But I'm guessing. Screens can display only about 70-80 dots per inch Paper can display five or more times that. In order to read from a screen you are forced to sit in front of an immovable fifty pound piece of machinery. You are less likely to use different postures than with paper.
Paper reflect ambient light.
Monitors generate it, paper doesn't. Can't say what difference this makes. That's all I can think of. Paper doesn't refresh at 70 cycles per second.