Much of this is the weight of copyright restrictions. There is a ton of material published between say 1940 and 1980, that is still under copyright but hopeless

Much of this is the weight of copyright restrictions. There is a ton of material published between say 1940 and 1980, that is still under copyright but hopelessly out of print. As physical objects, they're getting old enough that soon they won't really be available outside library/archival settings. It's many thousands of books per year for decades, lots of original research and writing.