1. Books are obviously composed of text, and, when you read, you're also imbibing the rhythms of the language you're reading (see http://www.newyorker.com/arts/critics/atlarge/2007/12/24/071... for more on this).
2. Related to number one, many more of us need to write than need to shoot and edit videos. Reading skill is related to writing skill.
3. You'll read far more words in a given block of time than you'll hear in video. Reading is much more information dense in this sense (whether a minute of video is worth a thousand words can be debated elsewhere).
4. It's easier to quote and store text than it is video. Notice the word "easier:" I'm aware that it's possible to quote and store video. Related to this, I use the scheme Steven Berlin Johnson describes here: http://www.stevenberlinjohnson.com/movabletype/archives/0002... .