Should? Says who? None of the fantasy literature I try to emulate features non-supernatural humans doing reality bending, physics breaking feats.
This conversation goes around and around and around but it's completely settled:
Different people have different base expectations/assumptions about their game world and what is possible. Done. Never the twain shall meet.
(For example, that's implicit in my first sentence: My game world is real world analog + magic.)
For me, it's not so much that the normal human adventurer isn't a thing. It's that high level D&D pairs that with some other expectations/assumptions that I don't really see going together often in fantasy literature and also doesn't have any narrative currency or other ways to bridge the gap.
Magic in fantasy is not often so reliable, versatile, no cost, powerful, and repeatable. Magic has more limits most of the time. Even so, contributing normal humans paired in small parties often have narrative help.