I was wondering the same thing, definitely seems like generating the raw waveform runs into all kinds of weird issues (like they touched on in this post). I would imagine that training data would be a serious chokepoint here. Given how much discourse is currently kicking off around the intellectual property rights of just the final product (the mastered track), I can't imagine many musicians would be eager to share what is effectively the "proof of ownership" (track stems or MIDIs).