Compare with the music industry, where there are a good number of streaming services, and pretty much all of them offer the same selection of music, all of it.
I don't think I know of anyone who pirates music at all.
The answer is greed. They make more being vertically integrated doing their own streaming than they would make taking a cut from a third party to host the same content.
I live in a place where people don't have money to pay for music streaming services, and even here, no one knows about soulseek or similars. What people do is listen to music via youtube. The only reasonably popular music piracy method around here is using telegram groups.