Because they are demigods. Mundane =/= Demigod. If you think Mundane = Demigod, I can't imagine what a super hero must look like to you!
Sure, they have magic. Martials don't have magic. Superhero-like powers, in D&D terms, is really just more magic most of the time.
D&D settings are typically highly magically infused worlds with fantastical elements everywhere (unlike the basically mundane world with a splash of usually powerful but dangerous magic which is also a trope).
Yes, mythic hero type abilities are "magical" when viewed by real world standards. But they don't have to be "arcane, divine, primal D&D magical". They can be intrinsic to the character which I think is what people think about when they think about mythic martial abilities. You can define this many ways -- divine spark, ki, just the natural laws of the world allows training to reach much different levels of power, born under a red moon, etc. Is it just another form of "magic"? If you are totally reductionist, sure. But clearly there is a demand for this different type of martial which I like to call "mythic hero" which is a "physical" type person with instrinsic (not using typical D&D magic sources) power that rivals what typical D&D spells can do.
What hasn't popped up in this conversation much lately is that D&D is a team adventure game.
I have no problem with having the slightly earth reality bending but not mythical "action hero" being a choice of character. This is a fairly common situation -- hobbits with people of power, black widow with Thor and Dr. Strange, etc.
A mythic martial hero should be also be a choice of character, which is also a pretty common thing.
The real question is -- how do you have both of those in one game, along side mythical Wizards?
You could have the game transition by level. So martials don't get mythical stuff until 11+ levels. If you want to play stricly mundane you stop at level 10.
You could have the mundane martials gain really powerful plot point / story points at level 11+ instead of the mythical powers , so they can bend the plot in ways that make them more relevant. When it comes down it, this is how it works in fiction. Curcumstances bend themselves to spotlight and make the mundane important and relevant.
You could also just have the existing mundane martials sitting alongside mythical martials and mythical Wizards and the DM just adjusts the plot to keep the mundane martials cool. Similar to the above but with much less agency as a player. (not one I reccomend but there are a lot of people on this thread that seem to advocate for this method vs. baking something into the ruleset).