I don't know who said it but I saw a description of this that made sense to me. In previous eras, American politicians had policy visions, real projects they were trying to see enacted. They could find successors and train them, introduce them into their patronage networks, etc. The modern era of American politics lacks any cohesive vision (beyond dismantling the system and selling it off to private owners), they don't have any vision of a future to share with younger politicos. There is no larger project they are a part of, they are all there simply for the personal power, influence, and the wealth that brings them. There's no compelling reason to step down if you don't give a shit what happens after you die, so you just ride it out.