I don’t like it, but it’s a pragmatic decision.
Hosting for a simple website can be as little as a few bucks a month. That’s easy for any project to absorb, even if they are open-source with no one pulling a paycheque.
Streaming requires high-performance, high-bandwidth machines that cost anywhere from several dozen dollars to several hundred dollars a month. You build a resilient high-availability network, and you could easily be looking at several tens of thousands of dollars a month.
That isn’t easy to absorb, even for a for-profit company with clearly-defined revenue streams.
Some people want everything for free, but free doesn’t pay the bills.
Full disclosure: I don’t use the streaming feature. I prefer to grab actual copies to drop onto my NAS. I also don’t share to friends and family, as I am the only one I know of who uses Plex.