On a serious note, why? These people (on both sides) have enough money saved, health benefits etc to comfortably retire. Why don’t they? Are they so bought and paid for that their ‘investors’ won’t let them?
When these people are strong and healthy they need to foster the next generations so that the party has a good idea who their successor should be so that there party doesn't fall into shambles and infighting every time one of them dies or retires.
The problem is that they don't do that. It is in their interest that there are no viable alternatives to them because good alternatives to them threaten their power. Until they are suddenly 80 that is and nobody knows how to replace them so they have to sit on their thrones and hold on for as long as possible.
Why should they retire? They don't actually do anything. They figure they might as well be there to soak up the corrupt funds instead of some other placeholder.
People who made it that far up are usually very driven, their job is their whole identity.
It's probably hard to walk away from something they dedicated so much of their life to. More so if it involved sacrificing time for relationships, family, friends, etc.
Seniority plays a major role in committee appointments. Sure, you could replace the senile 95 year old who doesn't know the cold war is over with a younger candidate, but that senile 95 year old has been in Congress for longer than the other guy has been alive, and he's on some important committees because of it that the younger candidate wouldn't be able to be in
Someone else recently explained it best: It's elderly abuse.
The entourage encircling these elderly congressional leaders are profiting greatly. As soon as these senators retire, these leeches lose nearly everything.
It's a Weekend At Bernie's movie plot, but with extra steps.