I don't see how you can compare being a senator to being a middle manager at some bank. The problem is that the banker faces huge consequences if they do something stupid to earn a little money by abusing their position and the upside isn't even that big. By comparison, senators have the opportunity to earn literally millions by doing things that aren't even technically illegal, like being given a wink-and-nod promise of a revolving door-style highly paid position in industry, "insider" trading that for some reason just doesn't count, etc.