I have a feeling that most of the difference is attributable to advertising.
Parents who play games don't care about ratings - they know enough to protect their kids to the extent they see fit.
Parents who watch a lot of movies don't care about ratings - they know enough to protect their kids to the extent they see fit.
Parents who read modern literature know what's in that too, and they know what their children are reading.
Ratings primarily serve parents who don't share that particular interest with their children. They're good for parents who don't play games or who don't keep up with movies. They only exist because there's significant demand for them.
However, the demand is only there because parents know that games and movies have content they find objectionable, and they know this because of advertising. Anyone, not just a gamer, can see a commercial for GTA3 or God of War 2. "My kid is playing this?" It wakes parents up to a threat to the mental well-being of their little ones, and they demand that they have some way to protect them that doesn't require them to actually take an interest in what their kid is doing.
Being poorly read is not a trait specific to the current generation. Parents don't read that much either, particularly not the fantasy/sci-fi genres that attract a lot of younger people. However, as is not the case with games and movies, book publishers don't run graphic advertisements for their products on broadcast television. Parents don't know what their kids are reading.
And, of course, reading -is- better for children than playing a video game or watching a movie. Society has a certain amount of interest in making sure that there's nothing stopping a child who wants to read from reading, and I have a feeling that a lot of parents would actually be more than happy to have their kid read something questionable - it's reading, after all, and parents hear little else but how beneficial it is.
I also think that there's a certain difference between a presentation of sex or violence in a movie or game, where nothing is left to the imagination, and the same presentation in words. While someone already familiar with the themes can be disturbed/aroused/etc, violence is generally much less visceral in print and sex, while recognizable as 'dirty', is not going to elicit the same reaction in an inexperienced fifteen year old as would hardcore porn.







