The bigger question is why has Congress turned into a assisted living facility for the infirm and mentally incompetent.
Easy answer: seniority is the coin of the realm in DC. For as much as we gripe about the likes of Pelosi and Feinstein, senior citizens on the other side like Grassley and Hatch need to go, too.
At least Orrin Hatch decided to retire. The history of the Legislative Branch is replete with examples of those on both sides of the aisle who stayed far too long, and profited oh so greatly, from "public service". Feel free to make your own list, but off the top of my head are: Teddy Kennedy, Strom Thurmond, John Dingell (whose wife now controls that seat), Robert Byrd, Thad Cochrane, etc.