Who Keeps Returning the Bums to Office?

David Bernstein asks a good question, Why should Larry Craig Resign? While he makes a good point, namely that other Senators have done far worse regarding their legislative duties -- a critique I'm pretty much in full agreement with -- one has to ask whey don't the other Senators resign, and I have to wonder why incumbents are such heavy favorites for reelection given how badly so many of them perform. Why don't we ever turn the bums out, and will the current catasprophically low approval ratings of Congress lead to fewer reelected bums?