I guess it's because of copyright, i.e. it's hard to make an illegal copy of a printed book, and it's easy to make a digital copy of an ebook. So the ebook-mark

I guess it's because of copyright, i.e. it's hard to make an illegal copy of a printed book, and it's easy to make a digital copy of an ebook. So the ebook-markup is partial due to the expected privacy. My other guess is, that it really is more expensive. I think, the publisher's digital infrastructure is really bad. And there is a lot of overhead when publishing a new book with lots of DRM on it. And most books don't get sold that often. So a small number digital purchases must pay for that over-proportional up-front cost.