The idea behind why certain folks keep their clearance after they've left the office that merited that clearance in the first place is a fairly sound one, at least as I understand it. It goes that former officials in CIA, FBI, NSA, NI, SD, etc. may need to be consulted by those currently in office, and during some periods, they may need to be consulted frequently. So, we can either refuse to let them retire from a job they no longer want to do full time (good luck with that), give up the advantages that kind of consulting can obviously give us, or allow them to keep their clearances.
In actual practice, it seems to get rather more nebulous. Once people figured out that companies will pay for advice from people who are privy to secret information gathered by our intelligence agencies, it became the norm for former officials to use the fact that they have clearance to make a fortune after they "retire" from public service...which usually happens after just a few years in whatever office they've held.