I imagine some are probably blackmailed or otherwise entangled with powerful people that wont let them retire.

Plus, being a senator ensures to a certain level that they can see what fuckery is coming to help protect their families and themselves.

Its power and prestige too. But its got to be a carrot and a stick.

Not to mention the potential boredom ,

for someone like that in retirement. Imagine spending most of your life traveling around the country, sometimes the world singing and meeting interesting people, amazing dinners. Actually working maybe a few months out of the year but most of the time just talking to people and playing a part.

And then one day , all that goes away , and you're just sorta sitting somewhere out of the game.

Lots of reasons. None of them good.