Politicians are just ambitious people, sharing full spectrum of other traits. Most people would become equally corrupt if presented the opportunity. Being ambitious just amplifies the temptation.
You underestimate the selection pressure. While I agree that many people would become corrupted, the profession (so to speak) attracts certain people. It is even to the point where you need some negative traits (explicitly: skeletons in the closet) to rise up, because it means you're easier to control.
> Most people would become equally corrupt if presented the opportunity