Other.
The way I see it, there are basically two reasons. The first is that groups often don't stay together long enough to make it to high levels. (As a corollary to that, high level games are much harder to play well and much less rewarding if you're not playing them from low levels to high- it's much harder to know a high level character's tricks if you haven't learned them over the lower levels building up to the higher levels.)
The second is that they just aren't well supported and haven't been since, frankly, 2e. 1st and 2nd Edition had a fair amount of high level support in the form of adventures, monsters in the base MM, etc. Since then, we've seen far fewer high level adventures, and most truly high level foes have fallen out of the basic monster books (when was the last time Tiamat or Demogorgon was in a Monster Manual? 1e).
Heck, even 3e, which had a whole book on epic-level stuff, didn't publish a single epic level adventure.
It's a bit of the chicken and the egg problem, IMNSHO.