You take an old copy from the public domain, invest time and money, make it beautiful, and republish it.
Now, your work is copyrighted.
Some other guy can take an old copy and make it beautiful, but not yours. That's protected by copyright. He would have to invest again. And then compete against you.
So it's even better than in the case of books. With books, after they are in the public domain, it's a free for all, very low barriers to entry. With stuff that needs to be restored, it's even more lucrative for the republisher, because they get a new copyright, less competition.
Also, note that this only covers the 70-90 years of undigitized stuff. Todays works won't need _that_ kind of work done to be republished.