So that, if no-one else was playing Halo until they were 18, we might avoid the argument that “Everyone else is playing it”.
In reality of course, it doesn’t work quite like that. Most of the nine year olds I know have played an 18-rated game. At a friend’s house. A friend with an older brother (although, unless the brother has some kind of growth hormone deficiency, not older enough). And they hanker to play more, because it’s cool.
Many of the parents I speak to argue coherently that if their child comes home with tales of playing Halo or Call of Duty, they then have to decide whether to a) ban child from going to friend’s house or b) let him go and wonder what it is that he is being exposed to. The feeling is often that they would rather the game was being played under their roof where they can see what is happening on-screen and monitor their child’s reaction to it.
Another all-too-common reaction is that age ratings are for ‘other’ people, that their children are mature enough and sophisticated enough to handle the images and narrative of an 18-rated game. (In my experience this attitude translates more or less directly as ‘I want my child to be cooler than your child’.) Have a look at http://wallblog.co.uk/2012/05/11/facebook-the-underage-question/ for a fine example of this - not necessarily the article itself which poses the question, but the comments below the line. Just the first one will do.