I'm sure they can come up with some kind of in-universe reason, but we all know that they can't really afford to lose anymore of the originals that are left.
Worf has the black mark on his record, Laforge has stated that he is happy where he is... But you don't always get to choose your postings. I'd like to see this addressed in the future somehow. I imagine that for awhile there won't be enough ships around to even need new Captains. After they start to seriously rebuild Starfleet though it becomes a harder question to answer.
Oh I agree, I don't expect
to see any of this happen just spit balling - but even without the ships - I would think that there are plenty of gaps in planet side postings.