The real reason for streaming is that most people don't know in advance what they want to watch, and with TV/movies, generally only watch something once.
I don't know if this is really true. How do you find out about something you want to watch? You see a commercial for it, or somebody tells you about it, etc. If all you had to do was pull your phone out of your pocket and click a button that says "download this when it becomes available" and then be presented with a list of such stuff the next time you want to watch something, you would pretty easily end up with more stuff to watch than time to watch it.