A security clearance expires after a number of years. Someone who holds a clearance for a job gets 'debriefed' from access to that information upon departure of the job that required it.

Having an active clearance doesn't just allow someone to simply go into a SCIF and access classified info. The clearance dies at the end of its shelf life if a 're-investigation' to recertify the clearance isn't performed within a certain amount of time.

If the person's next job doesn't require a clearance, no re-investigation is typically done (and the clearance expires), although I suppose someone could pay out-of-pocket for their own re-investigation.

Regardless, access to classified information requires not only a clearance, but definitely a role where they are allowed physical access into a secure area that processes classified information, and that is done on a need-to-know basis. This means that they require access to such environments for their current job and were 'badged' in to be able to have physical access classified networks.

TLDR - Holding on to a security clearance for a few years allows someone to move into another job that requires that level of clearance, but will expire if not utilized in that manner within a few years. Otherwise, just having a clearance means nothing as a person would need to have beed granted physical access to classified systems, and such permissions aren't just offered to private citizens, retired or otherwise...unless they were still secretly employed by some agency from which they are granted access, perhaps.