Because what you think is harmful may differ from what I think is harmful, and both may differ from what some government bureaucrat thinks is harmful. It's easy to point to extreme disgusting examples and say that those games are obviously harmful (which means it should be easy for you to spot them and forbid your kids to buy them without any assistance from the government).
But what about the games which are in a gray area? The ones which you think teach bad moral lessons but your neighbor thinks are fine? Or vice versa? Once you grant the government the power to censor things which someone considers undesirable, it's awfully hard to limit that power. Soon the government will start forbidding games from creating a hostile gameplace environment which disparages various ethnic, religious, and other groups. All games will have to be sanitized and politically correct.
This is not the path that people who support individual rights and a free society should want to go down.
So should we let everything be legal?
Since we disagree?
That is not true freedom.
Anyone who thinks this is a good idea, should look up the fight Frank Zappa put up against Tipper and the rest of the Washington Wives back during the PMRC junta. I think he refered to Tipper as a "cultural terrorist".
Bluenosed nimrods like Ernest Hollings (in reference to "bad" music lyrics, he said "If I could find a way to do away with them Constitutionally, I would"; and the absolute sheer stoopidity of the mischaracterization of Twisted Sister as a "satanic" rock band... it's humorous now, but back then, these people were serious.
And now we're seeing another ripple in the pond. Let Godvernment Almighty take care of it! That's the "answer"?