Come, you've watched the shows. You've a passing familiarity with TV. Most military based shows tend to focus on a small group of characters, even if the show is set on a base or ship with hundreds of people, whether not the shows writer/producers served is irrelevant to that. MASH, Hogan's Heroes and McHale's Navy did it. I'm willing to bet Combat and 12 O'Clock High did it too.
Yeah... how many MASH units in Korea really got by with just four surgeons? They did have a couple of other recurring surgeons in the first season, but they got phased out.
And you can see the same thing with detective shows. In real life, a homicide in a major city will be investigated by a whole team of detectives, and there will be different detectives assigned to different cases. But in TV, usually you just have two to four cops investigating every murder that happens in the precinct. In
Law and Order, you had the same two prosecutors always trying cases that were investigated by the same two detectives from the same precinct, rather than from all over the city. (And most likely an assistant DA in real life would have a much larger staff than just one assistant.)
Then there are all those shows that have one supporting cop character in the ensemble, and that cop is always coincidentally the one who's called in to investigate any plot-relevant crime. I see that contrivance so often (for instance, in
Charmed, Birds of Prey, and the 2007
Flash Gordon) that I'm surprised TV Tropes doesn't seem to have an "Only Cop in Town" page (unless it's under some title I can't figure out).
Even on
The West Wing, President Bartlet's senior staff was much smaller than the real thing would be, with the communications department consisting of two speechwriters and their secretaries, and with those speechwriters, the chief and assistant chief of staff, and the press secretary pretty much being the president's entire circle of advisors except on those rare occasions where a cabinet member or joint chief showed up.
And then there's
Mission: Impossible, which started out with the intent of being about one lead agent who assembled various different specialists and impersonators as needed for each specific mission, but ended up being about the same five (later four) people doing every single mission with only occasional help from guest experts.
It's just the nature of series TV that a show focuses on an unnaturally small and insular group of regulars, both because it would be too expensive to hire more people and because you want to focus on the characters the audience knows and identifies with.