It's not as lucrative. For human produced songs you can usually get the sheet music for them. If not, musicians can listen and do it manually, but it's not common enough to need AI to do it. Just transcribing for one instrument isn't that useful for many cases. Often they need an arrangement for multiple instruments, and depending on which instruments, the key may need to be transcribed. This is mostly referring to classical music and traditional western songs.

LLMs can do this well, though, and there are such. They weren't calling themselves AI when I last looked a couple of years ago, but I'll bet any of them looking for VC money have rebranded since.