My personal library serves as extension of my brain. I may have read all my books, but I don't remember most of the information. What I remember is where in my library my knowledge sits, and I can look it up when I need it. But I can only look it up because my books are geographically arranged in a fixed spatial organization, with visual landmarks. I need to take the integral of an arctangent? Then I need my Table of Integrals book, and that's in the left bookshelf, upper middle, adjacent to the large, colorful Intro Calculus book.
And once I have found the book, the static non-shifting text inside allows me find the right page and spot on the page. If books were uniform horizontal streams of text, there would be a dearth of visual cues for inside-the-book navigation. But books aren't like that. At a minimum, the paragraph structure creates large block shapes on each page, with different sizes, often unique in patterns. And inside each paragraph are words of varying size, sentences of varying length, and letters which dip down between lines here and there, all creating a look of their own. And most books also have figures, images, and tables, which provide marked cues for navigation to the piece of knowledge I possess. All these cues rely, though, on their fixed spatial placement within the book and on the page.
In other words - a large part of our memory is spatial. As anyone who read a "improve your memory" book knows, one of the classic methods of improving your memory is using spatial cues (i.e putting memories in an imaginary vault/palace etc..). in Ebooks however there is next to nothing distinguishing one page from another. Additionally the pages themselves change somewhat depending on your settings.
This is argued more scientifically in this article:
He says that studies show that smaller screens also make material less memorable. “The bigger the screen, the more people can remember and the smaller, the less they can remember,” he says. “The most dramatic example is reading from mobile phones. [You] lose almost all context.”
H/t - Both articles from The Dish - Andrew Sullivan.