There are so many examples swirling around my head, but perhaps this one really helps drive the point home.
I went to Origins, and I sat down to play my first ever game of Warhammer Fantasy. The guy running the game was excited, and it was just the two of us, so we got to chatting. He said something about why Warhammer was one of his favorite systems.
"No matter how strong your PC is, you can still be killed by a rabid dog in an alleyway."
DnD doesn't do that. It doesn't deliver that experience. It really can't. I don't fully buy the position I saw some time ago that every character in 5e is a regenerating Demigod, but if a DnD character encounters a monster, there is an expectation that they will survive and the monster won't.
And I know, "well in my game..." I know people use the gritty rest variant, and run with half hp, and send monsters of double the normal CR and all that, but when run in the standard manner, with the normal rules, a DnD party finding a werewolf or a zombie or a vampire is going to likely end up killing the monster. That is the experience DnD promises. And as part of that experience, your character is defined by two things, three in this edition, but since second edition at least, it has been two things.
Your Race
Your Class
Just like no one is complaining that you can't get a touchdown in baseball, no one is complaining that the game designed to be about Vampires and their political machinations focuses on Vampires. Just like no one is complaining that the Skip card can't be used in Poker, no one is complaining that a game about human's encountering knowledge man is not meant to know, in a desperate stop gap measure to take one more breath of air before we are dragged under by an uncaring universe, focuses on that story.
Because that would be to go against the very design of the game.
But DnD?
DnD is a game about a group of adventurers going on adventures, and those adventurers come from a wide variety of backgrounds and skills, and they encounter other races and creatures and fight them.
And the game has always, in every edition, offered a wide variety of playable races. You can say that there are more ways to play DnD, and you may be technically correct, I could make DnD a game about political intrigue and land management and run an entire campaign with not a single sword swung or spell cast. But at that point, I've thrown out everything except a handful of skills and the d20, and if I'm not using 85% of the rules, am I playing DnD?
What is the point of playing DnD to run an Arthurian, humans only, land management game? Or to run a miniatures mass combat game? I can imagine doing it, but why? There are other tools that serve that story far better than DnD, and you've thrown out everything that makes DnD DnD.