I think it's due to a couple of factors:
1. There are not that many people interested in photographers who release photobooks who are not instagram famous, thus there would be less people who would scan the books that that they do get and may not visit forums in which people ask for photo books.
2. Because a lot of photo books these days are personal endeavors, people may want to support the photographer, as opposed to large companies such as video game art books which people share much more. Thus, people would likely want to encourage others to purchase the books as well.
3. most books only have a physical release which may mean that good quality scans could only be done by ripping the pages out which most people would not want to do, especially if it's a limited release.