No, it's more like "If fighters cannot be impossible then why is it okay for other non-magical things, like giants, to be impossible?"
Differing amounts of suspension of disbelief. That's why.
Personally, I can go pretty far. I can have a guy being so supernaturally strong that he can try to grapple a dragon. I'm okay with someone consistently jumping down 40 feet and taking no damage (deliberate jump, rolling at the bottom, but still).
But, for whatever reason, I get iffy if the guy can jump 75 feet up. Or if he can swim up a waterfall. Or cut a mountain in half.
Are both sets, above, impossible? Sure. Equally impossible? I guess so, yes. But, still, I'm pretty much okay with the former, and not so much with the latter. Why is that? Just my personal take on suspension of disbelief. When I accept "fantasy" I accept "better than reality" in my characters, but when I accept "fantasy" I don't necessarily open the floodgates to all forms of fantasy, mythic and non-mythic alike.
Essentially, just because I allow The Others (the White Walkers) in my game, it doesn't mean I'm okay with a teleporting Rand going around erasing armies from existing. One I'm okay with, the other makes me roll my eyes. I can watch the Lord of the Rings movies and be okay with Strider tossing Gimli, but when I see Legolas, I roll my eyes. I can accept Achilles being immune to damage except for his heel, but when I see most fight scenes in Crouching Tiger, I roll my eyes.
Why is that? It's just preference. There's just a personal line that people have. So, that's why it's okay for me to have non-magical giants and dragons in my game, but not mountain-cutting. That's why I'm okay with jumping down 40 feet, but not an "incredible anime-esque leaping ability." Being as good as Batman (as compared to basically any anime ninja).
Really, this is why. People will come up with rationales to match their preference. See the Element Air argument in Underman's post a page back. It's just preference, and with fantasy, people can "logic" their fantasy to match what they want ("this works because of
X, while
C doesn't work because
X,
Y, or
Z doesn't allow it"). Trying to get someone to accept more fantasy (or most fantasy) into their game because they've accepted some already will really only work so far as you don't exceed their preferences. Past that, you've probably made them roll their eyes ("once per day I can destroy the sun!"). As always, play what you like
