Groups who play beyond around Level 11 want to do their own and don't buy higher level material, it is a consistent phenomenon across decades and different companies.
It was built, they didn't come, so it doesn't continue to be built. That's just the economics.
Any group that gets to Level 13...
You got your Big Bad and your stakes. The DMG has a lot of advice on Adventure building, specifically to take hooks like these and build it out for your players. Amd if you have made it 16 Levels together, the hooks that will be relevant won't be things the writers of the books came up with.
For the minority who will play to that Level, and for aspiration. Again, the people playing at that Level don't use pre-written Adventure material even when it is offered.
Per my post above on what the DM books offer for high Level play, the Bestiary has eight Tier 3 threats and five Tier 4...
In terms of this book, however, you are looking in the wrong place for the high Level support it offers: first look to the Bestiary, then look to not the sample Adventures but the Campaign outlines in each Gazateer.
The higher Levels serve two purposes: the first is honoring Tradition, same as other random features of 5E D&D thwt WotC cannot actually change if they want3d to.
The second is to provide an aspirational model of what a super powerful Wizard could be and do, even if a player never goes there...
I juat don't think there is a natural market for it,or one of the myriad TTRPG publishers would have tapped it.
OD&D capped at Level 10. B/X went to q4. Narratively, that area is juat the natural stopping point where people want to start over.
Brandon Danderson has a three generation (might be fourth generation by now) model for Epic High Fantasy in particular:
First Heneration: J. R. R. Tolkien
Second Generation: folks riffing off of Tolkien's fresh yake
Third Generation: Writers reacting against the tired tropes of the...
The Orc cowboy thing did not seem all that weird, if they are frequently pastoralist cultures in the "Badlands" as historically depicted in D&D worlds, it fits. The Axe beak riding Orc tribes of the Fallen Lands to the Southeast of the Silver Marches and tot he West of the Anauroch...could...