Are we even talking about a real phenomenon here, or just an urban myth?
I mean, which of our heroes or sidekicks really do know each other outside their professional context? Two of Picard's officers had a joint past, the others were strangers to each other, even if the unique Data and Worf of course had a reputation. None of Sisko's officers knew each other. None of Janeway's did. Of Kirk's officers, McCoy was the skipper's old friend; another friend was brought aboard by Kirk's request and promptly died in the pilot episode. And that was it.
Kirk was on first-name basis with one fellow captain, Commodore Decker. Picard, too, somewhat knew the skipper of another starship of the same class as his (Varley), plus was personal friends with another skipper (Keel). Picard's XO met a captain friend of his (or an image of him, anyway) once. Sisko knew Calvin Hudson, and came to meet him because of this very fact
. He also indicated he knew fellow captains but never met them. No friends of Janeway were in evidence. And that was it.
Kirk did not seem to be close friends with any flag officers, but he did know some professionally, and they knew him. Picard and Quinn were friends of sorts, but no connection beyond Starfleet was revealed; other admirals just had working relationships. Sisko did not have old flag friends of note. Janeway and her helmsman came from Starfleet families with flag officer dads. And that was it.
What was there to "explain" again?