We wont. Zombies look too much like humans.
It's the doublestandards of our rating system. On the one hand a game where you kill zombies gets banhammer, but on the other hand a game you play as a terrorist (MW2) and can decapitate living humans, not zombies (AvP), get the MA15+ rating.
Our country is so far behind the times with it's rating system and it's fast becoming a nanny state.
Hopefully some kids get this game and parents see how violent it is and it causes some pissed off mums to get vocal. It might actually help us get our long awaited R18+ rating.