They are but someone has to be in possession of the book and go through the trouble of digitizing it. Usually it’s museums, libraries, and private collections a

They are but someone has to be in possession of the book and go through the trouble of digitizing it. Usually it’s museums, libraries, and private collections and a variety of enthusiasts eventually upload from those sources to Zlib/libgen. Since no one cares about copyright there anyway it can be more complete in some genres than other central databases.