They're directly spun off from metal often. That's why they're called subgenres.

Cultures can't decide things, they aren't entities of themselves. They're collectives of people. By simply existing within the metal culture and literally any other culture, you are effectively assimilating, bringing in new ideas and perspectives. Unless your culture is literally only ever interacting with themselves. Which sounds pretty fucking bland.

What even is just "metal" anyway? It's kind of a useless term on its own. It's an umbrella, and all you're deciding at this point is what that umbrella does and doesn't include. Is Ozzy metal? Iron maiden? Metallica, slayer, Megadeth, anthrax? They all use metal in their name. They all sound distinctly metal, but they're also completely different sounds, from completely different subgenres that to say they're the same is ridiculous. And at the same time you show me one person who doesn't identify one of them as metal, and I'll show you someone who's wrong.