Even a fairly corrupt politician might believe that they are less corrupt than the other guy, and would be able to use the law as a weapon to get an even bigger slice of the cake. Or, still more likely, they would believe that "I get my fair share, you're a petty self-dealer, he is blatantly corrupt" and that the law would not be used against their perfectly open and above-board win-win deals, only against that other guy's filthy looting.

But that aside, as noted upthread, it doesn't actually require an amendment, it just requires the existing Congresscritters to do their dang jobs. Not holding my breath.