It's up to parents
My point exactly.
Yes, it's a bit of a generalisation, but sadly the law often has to cater to the lowest common denominator. You can't say "OK, kids aren't allowed to play violent games unless you're a responsible parent" because everyone thinks they're a responsible parent regardless.
I hate it as much as you do, but the government has to do something about it because a lot of parents are doing such a lousy job.