Perhaps useful to consider the implications of the reverse being true - ie everyone who’s ever served free to swan around military establishments at their leisure?
As for security clearances. Leaving the military surely counts as ‘exceptional circumstance’ or at least ‘significant change in circumstances’ -so you’d expect to have to re-apply. Otherwise the vetting chaps really wouldn’t be doing their job.
I find losing access a bit of a pain and losing the clearance was a pain too. But both are perfectly understandable.