There were tons of humans on earth claiming divine heritage... And we cannot claim to have frequent verified gods hanging out here, in a fantasy world with verifiable miracles why no?. Realistically sometimes this is an explanation of ego and "maybe" some competence. Alexander the Great was plying the greek tendency but the entire nobility of some countries are considered to be of some divine heritage or being the descendents of Dragons
But that's the whole trouble.
Myth, legends, anime, and superhero comics are the only real genres that lean into post level 10 martial content. Action movies sometimes but you see there is an argument over that in this thread.
So D& martials end up with a lot of Nos and no Yes. However the game with age away from the older fans as the younger ones will say Yes and influence design more and more until they themself are the official D&D designers.