>>149416762Adventure shows also require a lot of new designs per episode
One thing people don't really talk about is how unpopulated shows became since the 90's. This is partly due to how character design and art direction began being more of a major job over one that you could farm things like extras out to younger inexperienced cartoonists.
If you watch an 80's cartoon like GI Joe or Transformers and you see a city, you'll see a crowd full of background extras. Since the 90's, showing dense scenes like that is largely not done. If it's a city, it's usually at night or some time where you won't see so many people. If you see new characters in a show, they're usually intended to be reused at some point instead of one-offs that were common in older cartoons. A lot of the time, background characters were handled by the lowest rung of animators- since style consistency is more of a thing(and there's less of an emphasis on "realistic" styles that gave you some leeway with generic designs), there's more work(and thus money) put into even minor background characters now.
Bruce Timm, for example, got his start working underneath another artist in charge of background and vehicle work in the GIJoe cartoon.