The average TV show is 20-45 minutes, the average movie is 2 hours, and an average book can easily be 24 hours of reading. Over the course of a month, you would generally get more "products" from movie subscriptions than you would from book subscriptions.
Also, buying a month of movies/TV shows (from, say, iTunes) would likely cost more than the $8 that Netflix charges. Buying a month's worth of books might not be much higher than $10.
It's a sad day when someone sets the value of a medium by the number of titles you can consume in a day.