AvP's sole purpose was to make boatloads of cash (I'm not sure if it succeeded, I'm sure it was at least moderately successful) and the best way to achieve that wass by increasing attendance at showings, and the best way to do that wass to make sure that nobody, least of the young consumers far removed from critical thinking and oblivious to obviously low quality, could be turned away at the door. Hence, PG-13 rating. See also - Live Free or Die Hard, a PG-13 movie in a tired R franchise.
Games seem to run the exact opposite way. If you want to go for the big bucks, you go for the M rating, unless you're making a platformer. I can't remember the last time I played a really well-done T-rated game that wasn't on a Nintendo system (maybe Burnout 3). Whether this has to do with lack of movie-style sales regulation is not for me to say. Regardless, you're right that the industry is appealing to teenagers, but teenagers find those things which are supposedly meant for adults appealing.