had two things going on that, taken together, lead to Rayna's question.
One is that the show assigned Star Fleet ranks in roughly the same order that the regular actors were billed. So the top actors seemed to account for the ship's whole senior staff. There were some exceptions, when a small-part guy would have commander's sleeves, but that was rare.
The other thing is that the show generally needed all its top actors already at their stations as each adventure began. This saved time getting them into the story, but it also left an unseen senior staff who ran the ship "at night."