I think it's mostly ego. You're a powerful and successful person, the kind of type-A personality who can get all the way to the top of a fairly exclusive profession, and you wind up thinking you're irreplaceable. "I could retire... but I don't trust that the person who replaces me will be as good as I am."

They're not even entirely wrong in the short term -- the Democrats in particular seem to have been doing a really awful job of nurturing younger leadership candidates over the last decade or so. Possibly because of the geriatrics who run the party feeling (subconsciously?) threatened by the push they'd then face to retire...?