I think some go into politics to "do good". I have known a few of these personally. I think the ones who stay in Congress for decades get addicted to "being somebody". They can't imagine life without it.
I know a few folks at work who were like that. They were "somebodys" at work, when retired, they would be just Ol' Joe Codger, what's your handicap? My former boss is still there working. Why? Money? He'd have plenty.