I think the difference ultimately derives from the fact that music is something you listen to again and again, which enables a small fee per listen model, which

I think the difference ultimately derives from the fact that music is something you listen to again and again, which enables a small fee per listen model, which means that it makes sense to get it out there everywhere. But you're right though, the difference is stark. I don't think I've ever had a random tune I'd like to hear come to mind and fail to find it on Spotify, conversely I can honestly say that I have not once found a movie I've gone looking for on either Netflix or Amazon Prime.