I've actually talked to 4 ex-congressmen in my life. As in after they were out for a while. Well, 2 R's and 1 D personally in the 90's and 2000's and was with my Dad when he was visiting with another D in the 80's. None of these guys lasted too long up there and all 4 of them said slightly differing versions of basically the same thing.
That is, plenty of people, from both parties, run for office, want to participate in government, want to do whatever they think is good for the country or will make it better, etc., and go to Washington with good intentions. Then when they get there as the new guy, the senior members and party bosses pretty much tell them, maybe in more flowery language, but basically ok rookie, you're gonna do this and not do that, you're gonna back these bills but not those bills, you're gonna raise X amount of funds for the party every quarter or year or however they measure it, you're gonna vote this way on these issues and that way on those issues, etc., and if they don't play along, or get uppity and try to push their own ideas, or just not bring in enough donations for the party, they'll just shut them down. They won't get many co-sponsors for their bills and even if they do they'll see they never get out of committee. They won't get as much or any party funding for their re-election campaign, and are just left to wither. They'll find somebody to back that will toe the line.
Pretty damn hard to make any real difference outside what the parties already wanted to push anyway.
They have such a lock on the whole process and function it's insanity. And it is most certainly all about the money. BOTH SIDES.