Parmandur
Book-Friend, he/him
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.The why spend time and resources to write 13 new Epic Boons? If the only players playing those levels are homebrewers then don't waste writing those feats. They will just write feats of their own if they don't like the ones in the PHB 24.
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 threats, a full third of the stat blocks on offer. And these tie specifically into the Campaign outlines provided in each Gazateer that give a direction for high Level play and endgame scenarios for Level 20 PCs.