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

Old movies have been available on various "free ad-supported streaming television" for a while now, so I'm actually more surprised it took copyright holders that long to realize that Youtube also shows ads and doesn't require people to install some wonky app that might or might not be available for their platform.

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.

I've always assumed there was a lot more "more trouble [i.e. time/money] than it's worth" associated with putting up old content in whatever form. As you say, there are a lot of potential complexities and figuring those out for something that is never going to bring in much revenue may not be worth it, however fervent some niche fan base may be.

I don't have any expertise here but my assumption would be the studios have a better way to manage digital content and rights compared to previous. It could very well be they have content available, free of rights, that can be uploaded to YouTube for monetisation. As others have mentioned, there are effectively no hosting or bandwidth costs associated.