Why kids buy rated M games.... in Off Topic
Sex and nudity: $$$
Not really. Most games that sell have violence in them. Hell, even Pokemon is violent and it's a childrens game. The only other games that sell are racing games which are starting to become inappropriate for children because they like to crash into eachother and smash up the cars and such.
Better ways to make money? You do realize that a Nintendo game with Mario and that cast of characters in it costs the same as one of those mature rated games (at least when they first release, on the last gen consoles). It's not a better way to make money, just a different way.
People don't just buy games because they have nudity or violence, and if they do they're complete idiots.