Every entertainment medium has some level of prestige associated with it mostly based on how old it is, which is the primary reason book reading is venerated. As for the democracy comment, I think the logic there smart people read books and smart people support democracy therefore the more people reading books the more democracy support there is. This is obviously nonsense but it's really popular especially among people who venerate book reading in the abstract like this.

Reading a book is also a decent proxy for being able to hold your attention for more than 5 seconds.