Why not make it a law? Because there are far more important things to be spending enforcement tax dollars on than this. Wasting tax money to enforce the unenforceable won't help... getting people educated and out of poverty will have a far greater effect on violence reduction, as numerous government studies repeatedly state again and again (but are ignored because that doesn't make good shock-journalism).
#14- You are absolutely wrong. Getting an ESRB rating of M or higher *severely* limits your ability to market and retail a game already and has for years. The rating system is there and has effect, but just like parents who take their young kids to R rated movies (e.g. South Park, it's a cartoon, so it must be for kids!), can be ignored by the irresponsible.
The other real problem we have to make the current generation in political power understand is "video games" != "for kids".