because you would be distributing music that is protected by copyright.
Creating mixes is in itself harmless enough, provided they are for your own use. If you use them for public performances you should pay a performing rights fee, and if you want to sell them, or even give them away you must seek a copyright consent from the holders of the copyrights in every track you use - even if you only use a part of it.
That's the legal situation, and the fact that a person who downloads a mix may have 'legal' copies of each track in the mix has got nothing to do with it - they only have a copyright licence to play the music from their purchased copy.
Complicated isn't it?