Copyright has a lot to do with what we as a society want to protect and encourage. We want to protect an author that put the hours into creating a book, as opposed to the person creating a copy of that work. The person copying can claim they put in work too but the claim is not strong enough to override our preference to protect original authors.

Part of the problem with generated works is that it is lower effort like the person copying something. It’s not an activity that demands special protection like original authorship. I believe this is a large part of the reasoning.