Is this a D&D thing though.
Yes, a D&D /Community/ thing. And as D&D goes, so goes the hobby, for the most part.
But, it's not a fantasy-genre or myth/legend thing. Heroes in genre (not even limited to the fantasy genre, action heroes in general do crazy stuff as a matter of course, even in settings where the supernatural is not on the table) go well beyond the pedantic dictates of the community's double-standard.
And, D&D, itself has occasionally violated it's own community's double-standard, resulting in considerable controversy.
Can you name a fantasy rpg where the equivalent of a D&D fighter gets non-magical but reality bending abilities?
"Reality-bending" doesn't sound meaningfully different from 'magical' - it'd still be supernatural in some way, just with the hand doing the waving not holding a wand. Psionics, for instance, nominally 'not magical' but still supernatural - and, magical enough to slide on the double-standard in question. Like magic, psionics can do basically anything in D&D.
The ones that spring to my mind... Exalted, Earthdawn, Heroquest, and so on all actually use magic as the justification. In fact I find it curious that a portion of the D&D fan base wants a reality bending fighter but doesn't want the developers to use D&D magic (mainly spells) to empower the concept...
Here, you are clearly illustrating the double-standard. You cannot even bring yourself to articulate the concept a fighter not fettered by genre-inappropriate enforcement of 'realism.' You default to anything along those lines necessarily being 'magic.'
Of course, no one will be able to give you an example of a non-magical fighter performing magical feats non-magically, because you you're asking for a paradox. Not anymore than an Omnipotent God can create a stone He can't lift.
To facilitated the discussion, let's set aside the fraught terminology of - magic, spells, fighters, realism, etc - and settle on a clear definition of the kinds of abilities being excluded. Consider natural vs supernatural. For our purposes, here's a test:
"Is the feat in question the same in kind as an ordinary feat?" For instance, a man can break a rock with a hammer. Perfectly natural. Now, our hero walks up to a castle wall with his warhammer and smashes a breach in it that 4 knights could ride abreast through. That's impossible, it's superhuman, but it's not supernatural: it's still basically just breaking a rock with a hammer. Or maybe he simply leaps over the castle wall. Again, impossible, way beyond the world's record high jump, and he's wearing full armor. But, people can leap. It's like hopping over a knee-high railing, in kind, which is perfectly natural. So leaping over the castle was is not supernatural.
On the other side, consider a man without a hammer talking to a rock, and the rock shattering. That's not natural, even though it's well within the abilities of people to break rocks. In D&D, it's a classic spell, called Shatter, and you can use more powerful spells to take down (or put back up) a castle wall. Plenty of other things are supernatural. Mind-to-mind Telepathy (actual two-way contact with awareness of another mind, not just 'ESP,' which can be put down to intuition, cold-reading, and lucky guesses), Teleportation, de-materialization, ex nillo conjuration, etc...
Make sense?
When an ability in D&D crosses the line between accomplishing what people normally can and the superhuman, it generally gets a free pass /if/ it's supernatural, but it's likely questioned if it's not.
That's the double standard.
Heck, sometimes, even if a non-supernatural ability /is/ arguably within the realms of merely-human ability, it catches flack.
There's massive outcry if the fighter gets anything remotely non-mundane - even if it's just a nod to the abstraction of combat in D&D.
Best example I can think of recently was when the designers tried to put in some damage on a miss mechanics (ex. even if you miss you do your strength mod in damage) - the outcry was swift and massive.
Yes, like that. Hps are crazy-abstract, including all sorts of non-physical factors, that you might degrade without laying a glove on the other guy - indeed, that even a hit that did damage might do no physical damage is a point made in 1e, and re-iterated in a less wordy form in 5e.
