When I was at Spotify 3-4 years ago, one of the problems was poor metadata for most music. There's a semi-internal project to be better at classification (https://everynoise.com/) but it has its own problems.

And you can't rely on automatic classifications either. But then music providers don't give you proper info for the music they provide. It's a circular problem.

And then it's a social problem, too. You may want to listen to music that other people that listen to your music do... But then you might not want to do that.

It's a mess.