One of the most reliable tendencies of politicians is that they think they’re more important than they are. This is not a partisan point. It’s as true of Republicans as it is of Democrats, as a general rule — though it wouldn’t shock me if it’s more acute in more Democrats than Republicans for the simple reason that Democrats tend to invest more in the power and nobility of government than Republicans do. Politicians are prone to thinking the country needs them where they are (or in some higher office).
No doubt the reasons for this belief are complicated and numerous. Sometimes it may even be true. Churchill believed he was the indispensable man, and for a time he certainly was. But most of the time, I think it’s fair to say that this mindset is derived from a bundle of rationalizations. Politicians like being the center of attention. They like power. They like “being in the room where it happens,” to borrow a phrase from the song. And, often, they’ve got no place else to go, to borrow another from An Officer and a Gentleman.
Hence, they stick around longer than they should.