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

Free money?

It's probably zero effort to upload them to YouTube. People watch them. YouTube generates ad revenue and pays out Warner Brothers.

They probably choose the movies nobody wants to pay for any more on VoD/DVD and nobody views on paid streaming services.

I wonder if any of them will make back the amount of dollars the intern was paid to upload them.

I would say 1 million views should roughly pay the interns salary for a month. Some of the movies accumulated around a million views within the first 10 days.