Active vs. Passive Consumption
Here's why: we consume text in a fundamentally different manner from music & video. When I watch a video, I don't wait until I've absorbed each frame before choosing to push forward to the next. Instead, the media washes over me, defining its own pacing. Books, on the other hand, require engagement. I necessarily dictate the speed of my own consumption; I develop an understanding & perhaps a visualization of the action, then proceed to the subsequent segment. Marshall McLuhan said everything I'm saying here in a more philosophical manner with his spectrum that ranges from "hot" to "cool" media, arguing that movies, for instance, enhance our visual sense so much that the audience need not participate to fill in the details.
But the difference between books & music/video isn't limited solely to consumption. We are proactive with texts in other ways, too. When I read a book, I underline notable passages, jot notes in the margin, skip back & forth to contrast sections with each other. I pause to look up a word in a dictionary, or an event/personage/concept in Wikipedia. A good book is more circular than linear*, it becomes a conversation, it connects with other objects. And I would argue that the biggest failure of eBooks lies precisely here: they don't facilitate easy annotation or nonlinear navigation...yet. They will eventually, though. The "pause to look up a word" action, for instance, is fertile ground for eBooks and there are already browser extensions that provide definitions without interrupting the surfing experience.
I feel pretty qualified to talk about this. Why? Because I'm a librarian who hates paper. I cringe whenever someone hands me a printed email. I've tried to digitize as much of my life as possible. Yet I still read paper books. I'm not nostalgic, I don't love the smell, & it's not because it's a break from the screens that persistently mediate my life. It's because books are the best way to consume text right now, period.
*English major aside: my two favorite books are James Joyce's Ulysses & Deleuze & Guattari's A Thousand Plateaus, both of which are explicit about their circular/rhizomatic qualities (as opposed to linear/arboreal). Perhaps I'm wrong, but "plot writing" (as I think of it) is terribly boring. You don't read a book to find out what happens in the end, you read a book to get lost in the middle.