Because buying a movie or a game is much more intentional in the sense that it appeals to a specific demographic that has opted in (my grandma would HATE Half Life or any most rated R movies, but is way less offended by a sexual reference in a sitcom).

Mainstream Media doesn't want it, enter Showtime and HBO.

I watch movies and play games that I wouldn't want my six year old to be exposed to when he was flipping the channels? Like Leonidas in 300. My kid would have nightmares for days!!!