Because they're playing FATE, not, y'know, D&D...Are people playing FATE playing D&D?
If not, (given the argument above) why not?
Well, crunch and whatever. Mechanics are fine, I love making and using them, but sometimes it's more about the narrative tools, or a combination of the two. For example, in a recent Blades in the Dark conversion to 5E the author connected inspiration dice with the ability to use BitD style flashbacks, which I thought was bloody brilliant. Anyway, that sort of thing, just trolling for inspiration. Amber and Burning Wheel have been my light reading for that lately.....woah!
I have incorporated a fair amount of Amber mechanics into my D&D games to streamline the process, but it's more for rule of thumb, decision-making, flow. Not sure I'd use it for crunch.
Eh, that's fine. That's a conversation. I'll call your attention to what you wrote above:
If people talk about "playing D&D" and it turns out they're not using what is arguably the core mechanic of D&D (rolling dice), I'm probably going to feel as though they aren't, in fact, playing D&D.
You are entitled to your feelies, but things get, um, DICEY when you start telling people that "they aren't, in fact, playing D&D."
Hehe. Dicey.