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

Of course, region-specific copyright deals are incredibly complex etc. etc., so I could imagine it was just a matter of waiting out until the last person putting up a veto retired or moved on to other things.

Judging from the clunky, buggy, nonsensical experiences on 2nd tier streaming services (i.e., everything except Netflix, Amazon Prime, YouTube, Disney+, Max), I'd say the biggest cost is probably hiring a decent Engineering+Product+Test team. There are complexities here, like making these things work on different TV brands, versions, older models, etc.

Pushing all the complexity to YT seems like a total no-brainer.

1: Every customer wants their own twist. It is not enough to create an awesome video player app and reskin it, no they all want to be special.

2: Getting the last 5% takes twice as much work as the first 95%. Probably even more.

It's quite doable for 'normal' engineers to make a steaming platform. You need to get the video files out there on some CDN, you need some service for the DRM keys (which needs to scale, and handle the different access packages), and you probably want some history and profile stuff. Easy enough. But for the best experience you want every video to start playing in less than a second. That means getting those starting video segments as close as possible to the customer, it means optimizing that DRM key delivery, and optimizing the player so it just gets that video pushed to the screen ASAP.