Part of this is that books and screens are not interchangeable pieces of technology. Books are still supreme when it comes to reference material and lookup speed but each page needs static typesetting. Screens are more fragile, expensive, generally smaller, with lower contrast and/or resolution, but they allow a fully flexible display with variable font sizes, the ability to scroll half way between pages etc. A PDF on a screen is all of the downsides of books with none of the upsides of screens.