I don't think quality is really much of a concern for the majority of people, only enthusiasts. I suspect, analagous to 128 kbps opus (on youtube music), most people can't really tell the difference between a 1080p bitstarved stream and a 4k bluray rip.

The library for the music streaming platforms is much bigger than for films, of course (about 250 million for Spotify), but there's also a much lower barrier of entry. So perhaps the higher work needed to produce a film necessitates more profit, to a degree that only the fragmented streaming platforms can fulfill. After all, netflix started making originals to counter studios launching their own streaming platforms to raise profit margins, and pulling their content off netflix.