Several movie and TV sites have come and go where you can just visit and watch without even creating an account. Quality might not be as good as a paid service and they are no doubt in the grey-to-red zone legally. I don’t see the same service for music.
Why do you think is there such a difference in trends?
You look at the screen while watching video, making ads profitable (they obstruct the view making you click something to close it, hijacking the click and generating visits).
If you do it for music people walk away and nobody clicks on the ads.