Why is Warner Bros. Discovery putting old movies on YouTube?

I'm also a little surprised no one has yet (AFAIK) done the "viral indie release to Youtube" path. I feel like it's sitting there waiting to be exploited.

There's a lot of "indies releasing things to YouTube directly". However, they're limited both by the algorithm and by the amount of money they can generate by that, so you get a fairly restricted set of genres that this can work with, like sketch comedy or (perhaps a bit surprisingly to me) science documentaries, like Veritasium or Practical Engineering.

These are basically indie filmmakers doing a very indie thing that doesn't fit anywhere else.

Movies are, after all, as affected by their release technology as anything else. There's a reason they're all 80-130 minutes, and they have their own genre restrictions as a result of it, especially if you think of it in terms not just of binary possibility but how popular things are. It isn't reasonable to expect a very different distribution method to result in "movies" you'd recognize from the cinema any more than it is reasonable to expect that television would only ever have run "movies" and never developed its own genres that don't work in cinema. Taking into account the need for the content to match its distribution there's a ton of indie stuff on YouTube. What I would say you are really seeing is the restrictiveness of "The Algorithm", and that is an interesting question to ponder on its own.