Well, their constituents are happy with them and that is the bottom line. Every single election, I read again and again and again about how much everybody hates incumbents only to watch while not one incumbent ever gets primaried out.
And that's not the media's fault. That's not big money's fault. It's not a campaign finance problem or anything like that. It is entirely because when you talk to the constituents, you realise that they elected exactly the person they wanted to. Politics is still dominated by middle-of-the-road suburban voters who like safe candidates who are more likely to water down legislation than to create daring new changes.
And that is all there is to it. Term limits won't do anything except drain us of experienced legislatures and make us more dependent on lobbying. If you want a politician gone, you have to change their supporters' minds about them at least to the point where they prefer alternative candidates - and that is something that you guys are very bad at doing, to explain WHY these people are so awful. "But they're so corrupt, and they haven't done anything good, and conspiracies..." THAT'S NOT GOOD ENOUGH.