I guess you guys underestimate how hard it is to find music tracks that
A: Don't sound like complete shit
B: Can be used in a porn context
Even if the music artist agrees to the Dev using their song in the first place many of them turn around and want it removed once they find out it's porn.
Last week I went in a small horror themed park with my family and they played the music of the scene with Penny and the vampires. It stunned me for a bit haha I wasn't expecting that.
Downsides of royalty free musics I guess