Most games are bought for kids by their parents or older relatives. You'll find roughly the same percentages of enforcement on buying R rated movies at Best Buy etc. as you would on M rated games... and both are (and should remain) voluntary policies, not state enforced. And movie theaters do a far worse job of keeping minors out of R rated screenings than game retailers do restricting sales to minors.
The problem is that the older generation by and large still just does not realize that video games can contain adult content on a level of the Sopranos or Kill Bill. Does that mean we need legislation and government growth for it? Nope.