No one said the odds of corruption are less. But when everything is top heavy and centralized, the economies of scale for corruption are much better.
That is, for example, buy off a single Fed official vs buy off 50 similar officials at the state level.
And if someone tries that and one state gets exposed that's a red flag for all the other states to check their shit.
States are the equivalent of a diversified stock portfolio. The Fed gov is like betting it all on Enron.