I always wondered that. If there is some obscure reason that they need to continue clearance, then they should have to re-apply and go under HEAVY scrutiny.
Quite a while back I read that (supposedly) one reason is because they MIGHT be called to consult on some issue having to do with their previous job or position.
That sounds like more Deep State rationalization for people in the clique to receive special treatment.
We know that once they are in the club they are above the rules and laws that apply to us little people.
And this goes on for years after they leave the government.
The last I read, Hillary still had her clearance and she left the State Department more than 7 years ago.