You’re missing my point, which is that the “cultural factors” are not about individual people being inherently “corrupt”, but more about institutional strength and power distribution in the whole society. The US has a long history of integrating immigrants from a variety of cultural backgrounds, historically including large numbers of rural peasants from quasi-feudal societies, and plenty of people from places dominated by local “corruption”.

Individual people make up society. If those individuals are mostly corrupt, then society becomes corrupt as well, isn't it? If a judge knows that he won't get in trouble for accepting bribes because the DA won't prosecute, then the "institutional strength and power distribution" would be affected by that. Where is the difference here?

And not to nitpick, but why are you now scare quoting corruption, when before you did not? Do you not think corruption is a real problem?

If you don't have a legitimate court system or an accountable political system it's very hard to fight corruption, which makes it rather unlikely that either of those things will be established. So the reason for less corruption now can indeed be less corruption in the past.

So if less corruption in the past is the reason there is less corruption now, how does society ever get corrupt?