abirdcall
(she/her)
Beginning with level 12, it feels to me like nobody gets anything interesting (beyond new spell levels) until you hit level 18. Most classes just have "Hey, you know that thing you do sometimes? It's trivially better now or you can do it once more per rest!" and they stick that on the class table and say it's not a dead level. Yeah, that's still a dead level. Every time I see an ability like that I think, "The Rogue doesn't have +1d6 as their class ability every other level, and that's a better new ability than I'm getting right now. The Rogue gets a new ability *and* his existing ability improves." Maybe it's because I played a Rogue first. It just doesn't feel good to advance your character beginning about level 12 and lasting until you get to the capstone levels (18+).
They didn't put much effort into 12+. They designed the game around levels 5-10 (which is why they last the longest). Those are the levels that most people want to play in.
Levels 12+ also go by very quickly. There is less experience required to go from 11 to 12 than from 10 to 11 and with the huge power increases it means that the party will likely be fighting enemies worth a lot more experience. So yes, the individual levels matter less, but at that point the PCs are quickly getting to the point of breaking the game anyway.
I think most people playing 12+ are in it for the end game of handling threats to the multiverse at levels 17+. So 12-16 is like 1-4, just a bridge to get to the real game at 5 or 17. And once PCs are that powerful nothing is rooted in reality anyway.