Ooh, interesting idea. I had not considered it before and must admit I am not well-versed in the specifics of Youtube content upload with regard to music playlist 'mixes'.
I am under the impression one would need express permission from every artist involved, otherwise the upload would be subject to takedown demands. Spotify of course doesn't have that concern. For Youtube, that could mean a lot of work on the audio mix creation and upload wasted if any of the artists have aggressive content hawks.
Let me know if I'm incorrect on any of these assumptions, and thank you for the question!