Americans don't do it voluntarily, but mainly because they don't understand the way the US system works. Part of that is the nation's legal system; part of it isn't, but is instead international.
Regardless of whether or not today's United States is a democracy, our legal system possesses two features that make corruption especially difficult to prosecute to conviction, and this difficulty is extreme and makes such convictions extremely rare at the top, amongst members of Congress, and Presidents, and former federal officials, and billionaires, so that people at that level need to be extremely stupid in order to be convictable for whatever corruption they might do. (And such extreme stupidity is virtually non-existent amongst that elite, most powerful, group. So, they get away with it. There is absolutely nothing to stop them.) Consequently, corruption is rampant at the top in America.
Both of these two domestic, US, features apply also to some lesser extent in every other country; and, at the end, I'll describe the exacerbating factor that makes the situation especially bad in the United States the international factor, which intensifies America's corruption-problem.
Reason Number One why Americans favor corruption is that (especially at the top) corruption is, to a large extent and very unlike lower-class crimes of direct violence a judgment-call, largely political, and therefore specifically a partisan matter to judge. Anything that's "partisan" is especially difficult to produce a unanimous verdict, which is what would be required for a conviction. Consequently, an individual of high status within the elite will be granted by his group or "party" every benefit of every possible doubt. This is likely to protect any elite person from being convicted.
Take, for example, the latest "CBS News poll: Majority of Americans and Democrats approve of Trump impeachment inquiry". On the question of impeachment, 87% of Democrats approve, while 77% of Republicans disapprove. There's nothing even approaching unanimity. However, since impeachment is not entirely an issue which is based solely on the possibility of there having been corruption, a better indicator here is the poll's other main question, of whether Trump deserves to be impeached over Ukraine. That question concerns not only the possibility that Trump might have acted corruptly in this matter, but also the questions of whether Hunter Biden did, and of whether his father Joe Biden did. Consequently, it measures only on corruption (abuse-of-office), not at all on other impeachment-issues; so, this is an ideal measure, for our purpose. Of course, Trump is a Republican but the latter two are Democrats, and Joe aims to replace Trump if Trump doesn't become impeached by the House and then convicted in the Senate, and Joe aims to replace Mike Pence if Trump does become convicted in the Senate and replaced there by his Vice President. Though corruption is the issue on both sides (Republicans versus Democrats), it is an extremely partisan issue and therefore plays extremely to each side's political prejudices. 75% of Democrats believe that Trump deserves to be impeached over the issue of Ukraine, and 8% believe that he doesn't. 70% of Republicans think that he doesn't deserve to be impeached over Ukraine, and 16% believe that he does. Consequently, the single issue of Ukraine accounts for almost all of the beliefs on both sides regarding whether or not Trump deserves to be impeached, and this issue of Ukraine is virtually 100% an issue about corruption.
In a court of law, whenever a particular issue is politically charged, unanimous verdicts are virtually impossible to attain. Neither impeachment nor removal from office requires anything even close to any such unanimity as judgment in a court-case does, because neither the House nor the Senate would be voting anything like jurors do in a courtroom, and the rules are extremely different; and, so, any corruption trial that's in a court of law faces an almost impossible barrier against conviction (because the standards of evidence and the other rules in a court-case are far stricter). So, this is one way in which most Americans favor corruption. (Like everywhere, Americans are prejudiced i.e., partisan.)
Reason Number Two for this almost invulnerability at the top in America is that America's elite, even more than in other countries, normally possess the financial wherewithal to hire enough lawyers of enough excellence in order to crush the prosecution's case. Furthermore, there are a multitude of fine points in our laws that were written precisely in order to enable almost any of these people to be able to avoid being convicted. That's one of the main things they buy politicians for to get those types of "fine points" into the laws. It's how the American system functions.
So: not only are the laws full of 'loopholes' that were placed there especially in order to get such people off any hook, but, on top of that, these are the people who can and do hire the 'best lawyers that money can buy' and all of the lawyers and investigators that they need in order to get off scot-free or else with only slap-on-the-wrist fines and 'community service' in order to avoid legal perdition, no matter how corrupt they might actually be.
As regards the non-legal reasons why corrupt individuals in our system almost never go to prison, just consider what those 'loopholes in the law' really are: they are expressions of cultural values. Each loophole is argued for on the basis of some cultural values. Whereas those particular values obviously disadvantage the homeless and minorities and uneducated and inarticulate and ugly and poor (since few if any of those people hire lobbyists), the flip side of them provides advantages to the successful and the educated and the beautiful and the articulate the very types of people who are the likeliest to be corrupt. You don't find many of the elite people in prison, but you do find them in executive suites and Ivy League campuses and on Capitol Hill and in finance and in think tanks and in lobbying firms the places where power is wielded for the people who have the most money (who hire these people as their agents).
Whereas individuals who are homeless or minorities or etc. might elicit more sympathy than the rich and powerful do, they really don't have the laws on their side nearly as much as the elite do, and the elite are also vastly likelier to have the most competent legal representation and the legislators and judges on their side. But if ever a non-elite person is corrupt, that person is extremely likely to "have the book thrown at him" and to get no sympathy at all from the public. This exemplifies the core principle of conservatism: all praise goes upward, all blame goes downward.
And, so, anyone who supports this system is actually favoring corruption. They do it not voluntarily, but instead because there are things they believe and don't even question such as that the elite are superior but that are actually false. So: corrupt people almost always get away with it.
The reason, why the above-stated reasons apply increasingly and especially in the United States, has been that after Russia ended its side of the Cold War in 1991, America's elite the most corrupt part of society increasingly and especially acquired global immunity for its violations of international laws, such as by increasingly invading countries that had never invaded nor even threatened to invade the United States. The 2003 invasion of Iraq made especially clear, to the entire world, that America would even go so far as to order UN weapons-inspectors out of a country in order to bomb it. That brazen and effective termination of the applicability to the US (and to any of its allies, America's vassals), of any international laws, constituted the making-public that the United Nations has been diminished to being little more than flapping mouths, at least since 20 March 2003.
Consequently, with no international body to restrain the American elite, the lid is now entirely off to corruption in America; and, the lower that the global public esteem the UN, the worse that America's corruption will become (if it's not already entirely free of restraints). The restraints of international law (such as whatever restraints had previously existed) are now perhaps totally gone. Consider, for example, what happened to Gaddafi, and to Libya, in 2011.
A big change in global public opinion toward the UN occurred as a direct result of the US-and-allied 20 March 2003 invasion of Iraq. The only periodic polling that was done internationally on the public's esteem for the UN, and that covered the period both shortly before and shortly after America's invasion and destruction of Iraq, was by Pew. It sub-headlined "UN Less Popular", and reported that in the 15 countries where public opinion was sampled both in 2002 (before the invasion) and in 2003 (after it), the favorability rating of the UN declined in 14 of the 15 nations, and the decline was sharp in each one of them (declining typically by a third, during just that one year). Only in Pakistan did the public rate the UN more favorably in 2003 than in 2002. Only in Pakistan did the public approve the UN's becoming effectively nullified, and the US Government and its allies thereby taking over the world as not only the lawmaker, but the judge, jury, police, and executioner, for all nations the government of the world (a dictatorial government outside the United States, since the US Government cannot even claim to democratically represent any of those foreigners). And yet, only 13% of respondents in Pakistan approved of the United States in 2003, which was exactly half of the 26% there who approved of the UN.
The solution, when there is no legal government that stands above the nations of the world, isn't so much to make the most powerful nation (the US) less effective, as it is to make the UN more effective. The problem here isn't actually the UN's failure, so much as it is the US regime's freedom to violate international laws especially the UN Charter. There is clearly, now no legal government of the world's nations. The UN is less of a world-government now, after 20 March 2003, than it ever was before. So, international bullies such as the US Government reign with impunity. Look, for example, at what such bullies are currently doing to Iran and to Venezuela and, to Julian Assange. Ever since 2003, the international law-breaking has become blatant, and unashamed sometimes even bordering on boastful.
The longer that this international immunity of the US and its allies continues, the more corrupt America will become. FDR's intentions for the UN were correct; Truman's have by now failed utterly; as a consequence of which, the United States is effectively lawless at its top. A country like this, where the courts effectively trash its own Constitution, and the nation's executive sometimes even flaunts his flouting of the little that still exists of a Constitution for the world, stands in sharp need of an international force that can effectively say no to its government. FDR was correct about international law, and not only about the US Constitution.
One of the reasons why the UN has failed is that it has never clearly defined even the most dangerous international crime, "international aggression" (the invasion by one country against another country that had never invaded nor seriously threatened it), much less established penalties for it. The Nuremberg Tribunals after WW II were supposed to start the process, but the effort just faded soon thereafter (under Truman). Furthermore, existing international law is totally irrelevant to non-state aggressors, such as Al Qaeda, which should be allowed no protection by any government. Nor does international law address under what conditions a nation whose government protects terror-groups (such as Afghanistan, prior to 9/11) may legitimately be invaded (such as by the US after 9/11), nor what limitations ought to be placed upon such a retaliatory invasion. "Terrorism" itself is undefined. In other words: the UN, to date, is almost a total failure. When the US Government steps into this legal void so as to impose its will in flagrant disregard of what international law does exist, this reflects not only the failure of the United States, but the failure of all of human civilization, at the present stage. That's where we now are. Almost every international institution that the US set up after WW II needs to be replaced. We've been on the wrong path, since FDR died. And America has been leading the world on that wrong path. It's therefore no surprise that Americans approve of this path Obama's "the one indispensable nation", or Trump's "America first, last, and always" more than the world's other nations do. (Of course, Hitler, Hirohito, and Mussolini, also held that view, which isn't patriotism, but instead nationalism specifically, supremacist nationalism.) This has nothing to do particularly with Trump. Obama was perhaps even worse. It has to do, instead, with what America has been, ever since FDR died.