I think the main issue is that high level play gets too far divorced from reality. Sure, there’s campaign burnout, but really, I think most people want to play in reality (material plane) or tweaked reality (faewild, shadowfell) but to do high level play, you really need to leave those confines and go SOMEWHERE ELSE. Because, while things like giants and dragons may exist, for the regular world to make sense at lower levels, they have to be rare and hidden and then it’s really kinda logic breaking to have mooks for a 14th level party because why don’t those mooks just take over the world. So they’re buried deep in a dungeon, in some artificial construction, or only revealed in some world breaking occurrence. High level play has to just be boss fight monster hunting in any sensible plane.
The Other Places where high level play makes sense, The Abyss, Nine Hells, or Astral Plane don’t make common sense for the average player. Ok to visit, but not part of their character’s reality, or a place to stay and adventure in, too other. But those other places are where plenty of high level mooks can exist.