It seems a bit of a stretch to blame copyright for this.
People upload content for which they don't own the copyright all the time.
In this case, the people with the videos just don't want to go through the effort of digitizing and uploading. Even if the studios said, "All of our old stuff is public domain," it wouldn't change anything.
I think it would. If the studios would say that, they could have the local film academy students in to digitize; they like that and they can study stuff which hardly anyone saw before. They did that in the Film museum in Amsterdam and that worked fine. So yes that would work.
People upload content for which they don't own the copyright all the time.
In this case, the people with the videos just don't want to go through the effort of digitizing and uploading. Even if the studios said, "All of our old stuff is public domain," it wouldn't change anything.