That doesn't explain why a 20 year old movie will pop up on a streaming service for a few months then be unavailable again. Clearly they can sort these issues out. The content companies are stuck in a model of artificial scarcity.
For myself, when I get the itch to watch an old movie and I can't immediately find it on the Netflix or Amazon prime video I don't go try to find some other streaming service or pay for a one time viewing. I'll just find something else to watch that is available. So when they could have had one more view for old content they wouldn't otherwise make money on, thereby increasing the value of their catalog, instead they get nothing at all.