I've repeated this a lot lately, but fairly recently WoTC informed us that, per their tracking (DDB?) 97% of campaigns end before level 10 (someone please correct if I've got it wrong, but I'm pretty sure that's the gist of it, and the number was 97%).
That said, this forum skews older and I suspect has a lot more folks who have played or currently are playing in longer term campaigns. And I further suspect that that minority who play longer campaigns or upper level campaigns are probably disproportionately invested in their games, and probably represent a small but important subset of the overall pool of players. So we are likely worth catering to. That's all supposition, though.
Then there's the aspirational aspect of the game. Even if most characters don't reach those higher levels, it's fun to think about and even plan for it.
Overall, though, I think the game does mostly cater to lower and mid-level characters. For example, if you sort all the monsters in the new MM by challenge rating, there is a precipitous drop off after mid-level challenges. And there are much fewer campaigns aimed explicitly at high level play, such as the recent Vecna one.