There's like three different answers given here, and none of them mention distraction, I find it functionally impossible to read for study on my phone or computer for extended period without getting distracted even by things you wouldn't think of as distractions. Notifications distract, the ability to play music at all, the software that I'm reading on, highlighting text as I go, I just do not have a brain that makes it easy to read on a computer, and I think a lot of young people don't either.

e-readers and physical books just don't provide as many possibilities for distraction.