Maybe my opinion on this is negative due to my Eastern European upbringing, but I would say it's because it's something often seen as painful (think of a thick book with a dense vocabulary, being more scary rather than appealing to the majority of people), and, people are OK with wasting time if it involves pain, it gives the impression of a more honest attempt to achieve something, and books are the go-to for the majority of people who judge whether somebody is being productive or not.