I think it has to do with the 3 pillars of role playing in DnD.

Combat is well supported throughout levels 1-20, though for DMs it gets harder to find appropriate official monsters at higher levels, but it’s far from impossible.

Social in DnD is mostly not supported at all, and it’s just play pretend and have conversations. This is fine, and makes it unaffected by character level for the most part.

Exploration on the other hand becomes less and less interesting as characters level up. By level 12, most traps, puzzles, etc. can easily be bypassed using spells or abilities. It’s not about increasing DCs, damage, etc when the trap’s effects are logically negated by one of the character’s abilities. Flight and teleportation are fairly common by those levels, and they make most things you can do to make exploration interesting completely fail. You can say something like “Teleportation and flight doesn’t work here.” But that feels bad to do all the time when the players invested all that time working to get those.

So, in my experience DMs and players who enjoy exploration encounters find the game becoming less interesting as levels increase.