Probably because unless someone is sticking to the best sellers lists, there a ton of books to be covered, so it would lead to many individual posts about books others would likely have to go out of their way to read in order to converse on. Most movies, shows, and games are mainstream for the most part when discussed.
This scenario reminds me of my love for the IMDB forums before they were shut down, you could watch the most random movie, go to the imdb page, and there would either already be existing threads for the movie or you could create one and come back to a discussion over time. So in that case, you are probably better off finding a site dedicated to that kind of thing.