It’s a language so that people can discuss varying levels of nuance with each other. You probably don’t care about the difference between industrial metal and folk metal, but metalheads certainly do. And even that’s a pretty top level example.

Same goes for every other genre/subgenre. Indie pop and bubble pop can be very different, and just because someone likes one doesn’t mean they’ll like the other.

There is also a historic factor in all this because genres are always influenced, derived from, or mixed with other genres. Triphop was born this way, as a mix of hiphop, jazz, dub, electronica, and others