I'm not sure that this is accurate. You can take a look at IMSLP and see how much music there is that's in a more "popular" style (while still not anywhere close to modern pop, of course). Especially vocal music (most commonly songs) and instrumental dance pieces, which were the most common choices among 19th century amateur performers playing music in their home salons and parlors. It takes a lot of work to accurately transcribe the music from score notation into a form that AI can be trained on, but that work too is happening via e.g. the "OpenScore" effort. So I think we can expect better AI-generated notated music in the future. It could become a nice "toy" domain for sequence modeling (including language modeling) architectures, where high-quality results might be achievable with very little training effort. A bit like MINST is today for image recognition.