Trust the Fungus
It ain't so much that people don't want to play at high levels or have content for high levels-- though it's obviously going to be less popular than beginner and "sweet spot" content-- it's just that as games run longer (and reach higher levels) prepackaged adventure modules are going to be less and less relevant to them. Characters are more specialized, characters have more history with the specific NPCs of their game and even the NPCs of previous modules aren't the same running game-to-game because umpires run them differently and over the course of an AP they've changed in different ways than in everyone else's game who bought the same AP.Like practically nobody bought or played the Epic Tier modules in 4e, and similarly, the Epic Characters Handbook in 3e was a major underseller.
No way around it. The longer a game runs, the longer any game runs, the more homebrew (even if it's just story) it incorporates and the more loadbearing the unique personal elements of that game become.
It's not so much a lack of content as that tables tend to start at lower tiers, and burn out before they get to the top tiers of the game. Even if their tables didn't burn out, they tend to slow burn and advance at a lumbering pace that stretches out for years before even breaching level 10.
I'm not saying we shouldn't have Epic Boons or higher-level-locked feats, mind you -- I'm explicitly in favour of more epic tier content! But I don't think the rationale is because they don't focus on it. And I think they're tabling most feats for the time being because this package was already huge. The only feats included were the Fighting Style feats -- which are required to make certain classes work -- and the ASI feat, which is required for all classes in lieu of any other feats.