@givemersspls
The other people's points aside, the starving artist issue is still a problem in Japan. I'm not sure about manga in particular, but anime faces tons of newcomers leaving year after year because it's too taxing for the low pay they're getting.
And as an already published author who was top of the rankings, it would be unbecoming to step back into what's essentially an internship type role. Can you imagine say, Hiro Mashima becoming an assistant after Fairy Tail's serialization ended? You'd think, "Ah, what a loser, he's really washed up," right? It would basically signal that you didn't have the talents or the option to continue as you did before.