One of the top two reasons? They have an emotional--and tactile-- attachment to physical books. Turns out we all love to clutch those suckers tighter than Daryl cradling dat crossbow (though, for real, I'm trying to clutch DARYL).
And--and!!--they, too, like to sniff books. Cuz wood-pulp plus ink plus glue breaking down over time = the nose-noms :D And, lo, how thine noms art redolent of vanilla.
(Though I'm partial not just to old book smell, but new book smell, too. Fresh binding glue or old-ass lignin, I'm an equal-opportunity page sniffer.)