Well, for one, we're a country founded on not regulating things. There's that. Responsibility of the consumers and whatnot.
Second, what makes a game childish? I'd say that simplistic plot would be one of the main points. Bad guy kidnaps princess, hero rescues her. With mature rated games, usually the plot revolves around obscene amounts of sex and violence with little character development. Many people develop a conceptualization that if people think badly of what they themselves find entertaining that they therefore now think less of the person who indulges in these media.
Look at other forms of media where simplistic plot draws a greater audience than deep philosophical ones. People thrive off of the cruel criticisms of Simon, or raw violence of "professional" wrestling, or even the blunt insults of Dr. House. Cartoons are perhaps the epitome of this, with more volume of straight-forward violence (DBZ, any superhero cartoon) or slapstick humor. (All the old cartoons everybody has such fond memories of.) Even though they are more successful largely, it is seemingly much harder to put out an entertaining and more intelligent programs, such as Planet Earth and Seinfeld, or a cartoon which teaches morals, like Dr. Seuss. Twilight books outsell several classical philosophy works combined. People are on the whole simple and easily amused, yet fancy themselves sophisticated.
Finally, I would say that art direction often downplays the maturity factor of games. How many of them spend more effort pandering to male genitals than showing true figures of humans? It is rare for a Portal to come along that stars a female character without utilizing jiggle physics.
Games will remain largely immature until the maturity level of the audience changes. Regulations will not really effect this without sinking a large portion of the industry.
