The difference is that there have never been elves, and there's never been magic, in real life. But there have been real-life examples of terrible tortures, mass killings and mass rape (and not just in medieval times).
There have also been real-life examples of the laws of physics making themselves known, for example by chunks of ice melting when the climate is not cold enough. Yet, for some reason The Wall does remain standing, despite apparently being within a short walking distance of land with bare grass.
There have been examples of difficulty finding food in very cold climates, and the fact that such lands have very little people, yet (even if the onehundredthousand figure is exaggerated), the wildlings gather a massive horde, which somehow manages to find provisions.
There are medical limits to what amounts of torture people can take without passing out from pain.
So not only are there unrealistically exaggerated depictions of violence. Even if the violence were depicted realistically, the work is very selective about realism, and ignores other, physics and geography related aspects of it. The question is whether you get to be that selective, while claiming to be realistic.
Still, I admire George R.R. Martin's work for its drama. These things do not bother me that much. At least the depictions of people (and their relative skills in various areas) are generally consistent, which saves me from certain distractions to suspension of disbelief. The factions are also well plotted, overall.
Yes, these terrible things exist in the real world. But to focus a story on them to the near exclusion of anything else is no more realistic than dwarves and elves. Treating real things unrealistically does not make a story realistic.
The trouble is that the authors still choose to unrealistically depict these things, with barely any hint of consequence outside the acts themselves. They choose to focus on the terrible, exclude the mundane, and treat any sense of decency as anathema as dwarves and elves. To go a bit sideways here, these worlds often strike me as the hyper-religious person's nightmare of a world without their religion. Everyone runs around murdering, raping, torturing without one whit of conscience, decency, or regret. Which, in itself is an absurdity.
Pretty much this.
I would point out though, that I wasn't thinking solely of the grimdark subgenre, but any dark events in any speculative fiction (in cases where the dark outcome is not very believable, obviously). Even in heroic fiction, heroes are sometimes facing hardships they shouldn't.
My point was that unrealism can go both ways, by a work being too hopeful or optimistic, or by being too pessimistic. The coin has two sides.
Realism would also mean that heroes get matched up against weaker opponents and earn easy victories sometimes, as well. And sometimes heroes are challenged when they should not be, realistically.
