There are a whole lot of books that are out of print, and if a book went out of print before ebooks were a thing, it probably doesn't have a legal digital edition either.
This. Few people here would remember ebooksclub/gigapedia/smiley/library.nu [1] which predated LibGen by several years. But that online library had a lot of books that are not availble nowadays. There were lots of scanned books (djvu) that people uploaded. So much lost knowledge.
[1] https://en.wikipedia.org/wiki/Library.nu