I guess I’d flip that question. Why do you think being career politicians gives them motivation besides bribes and money?
Because that’s the thing, they know they’re running another campaign in a couple years, they always need to be raising money for the next one. They always need to solicit donations. And they can’t do anything that rocks the boat because it affects the next election.
Presidents very commonly get more done during their second term because they aren’t worried about the political impact of their actions affecting their ability to get elected again. I don’t see why this effect wouldn’t be the same for Congress and the Senate.