They’re not even main cast members. They were day players, they were as likely to get a character centric episode as Sulu or Uhura in TOS. I accepted that fact very early on. Just because Tom Paris was a helmsman doesn’t necessarily mean every helmsman from there on must be a main cast member that gets character development, BECAUSE OF PRECEDENTS! Every show kind of shuffles bridge positions in terms of main cast members, recurring, dayplayers, extras with no lines. ENT/TOS had communications officers, while TNG/VOY didn’t, requiring their tactical officers to do double duty in that regard. It all kind of fluctuates depending on who the writers want to focus on and what their assignments are.
In the instance of DISCO, our main characters starting in S1 had the traditional roles of Captain, XO, Tactical, Science Officer, but with the addition of a “specialist” and a Academy cadet filling various roles, which are atypical positions. As a result, a lot of the traditional bridge positions and Chief of Engineering were not played by regulars. Culber was CMO, but he wasn’t a main cast member in S1.