Digital textbooks frequently come with expiration dates. So you pay almost the same amount as a paper book, only for it to disappear shortly after the semester is over.

Other digital books cost almost the same as the paper edition. And with DRM frequently baked into the book, you can't pass it on to the next person.

That said, I am somewhat addicted to erotica on Kindle because the books that scratch my itches aren't available in dead-tree editions.