Why should any congressman, senator, government official, or even President, retain their security clearance after they are out of office. What would they need it for?
Given the cost and time required to get the clearance it probably makes sense for them to keep it for a while.
As others have stated, clearance doesn't mean access. They would still have to have a need to know and the party giving them the classified information would have to have a good reason.
Brennan claims that he hasn't been making any use of his clearance.
In fact Comey and McCabe both had their clearances deactivated when they left the FBI but they weren't revoked.
Many people who leave one position in government later take up another one. Maybe when administrations change or maybe they get called back to service - look at Mueller for example.
It wouldn't be practical for all of these people with knowledge and experience to have to start over at square one.
Leave aside the interest the government has in having key people flow into the employ of the defense contractors. We need experienced military and intelligence people, with their clearances, in the private sector.