I don't understand why people keep voting them back in?? They've been useless for so long, yet people continue to check the box next to their name. Why?
Some but not all of the reasons: It's virtually impossible for a primary candidate to unseat an incumbent in their own party, and gerrymandering makes it so that opposing parties rarely have solid candidates.
Same for there simply not being another candidate most of the time. If you have a 70% conservative / republican county or state, running democrat doesn't make much sense, and is an especially bad investment for any donors.
You might call it regulatory capture of democracy.
One big reason is that many people will insist that their representative doesn't contribute to a problem, or has a particular reason for doing so that excuses the behavior. It's essentially always everyone else's representative that is the problem, and so they vote theirs back in and the cycle continues.
Welcome to "democracy," where college studies show that only the rich and powerful ever get what they want while the normal citizen is continuously shafted in new and exciting ways.
Apathy and groupthink, my friend.
Mainly because of the two-party system. The pre-selection of candidates forces you to choose between two evils and it most often becomes a matter of the devil you know versus the devil you don't know.