Most of their forces are backgrounded. You can see that there are troops operating and carrying out attacks beyond that chain of command. I think it was more of an issue of budget, the same reason why most of the "kingdoms" besides Perfuma's are like 4 or less people.
Doesn't Shadowweaver tell Adora in the first episode that the rest of her training group aren't ready for the field? So the actual structure would be:
Hordak > Second > Force Captains > The squad of soldiers each Force Captain is captaining.
And then separately: Shadowweaver > Drill Sargent (you see him briefly at the start of episode one) > Cadets
You just don't ever see anyone besides the main people.