Why do almost all editions of D&D fall apart around 10th level

[Morty]

D&D can't accommodate different power levels. That's the whole ticket here. It claims to do so, but it doesn't. Different editions have different breaking points, but it always happens. "The DM needs to decide" is a non-answer with about as much meaning as your average marketing slogan. The person running the game always needs to adjust and make judgements of some sort. But the system is supposed to cooperate with them on it.