I'm afraid you have to chalk that up to the cliches of storytelling, like Captain Kirk leading every away party or The Unit's members being able to blend into every enviroment, the series wanted to explore every aspect of the war but keep the continuity of the characters
I'm reading through this thread since I am nearing the end of watching Space Above and Beyond, but I find myself wondering what Star Trek would be like if it were more like real life command structure:
Star Trek opens showing the bridge. A situation is discovered. A landing party is created. Rather than risk people like the Captain, Science Officer, or the Chief Medical Officers, all of which Starfleet has invested years of training and development, subordinates are chosen.
Actually I liked the episode of Star Trek Next Generation when Wesley Crusher was given the assignment to pull together a landing party. He evaluated the facts, looked at the possible people and worked from there. No command staff sent on that mission.
So in this setup you could have some repeats, but you could also have new actors and actresses get a start as the show would feature all kinds of new faces. Many short stories arced together by a common command staff that in of themselves are changed out from time to time.
On the grimier missions the Captain would be negotiating from orbit while the landing party works things out. Some landing parties might actually get completely wiped out (which we saw on Star Trek anyway.) But in some cases, people other than Kirk can show intiative and be promoted through the ranks. Disciplined, etc. A lot more lower decks in this format.
And naturally if there were meetings of high ranking people on the planet, the Captain and other high ranking persons would attend.
(Come to think of it, bring it back to Space Above and Beyond, that was the cool thing about COLONEL McQueen being grounded, to be prevented from being another Kirk).