I mean, zombies also aren't a thing, are you going to get up in arms about that?
There's an argument for the caliber of the rounds, and we could also argue that the skull is harder than ligaments and smaller bones in certain extremities, and we could probably also guess within the fiction that the virus does something to the body to make the head tougher and certain other things weaker. We have also seen that the virus sometimes shifts the control center away from the brain too.
There's lots of logical argent's you could make around real life constraints and in fiction if it helps you get there. At the end if the day they wanted a design that was more carried than just shooting a zombie in the head, and they succeeded in that.
It seems they just couldn't get around people's preconceived notions of how a zombie should act. That's probably why Dead Space 1 and 2 we're so good. They totally redesigned their zombies so people could accept the limb system there.
I'd argue that doing this sort of thing on existing zombie designs makes it feel fresh and scary in a way that zombies typically aren't.
And it still stays true to the original game to an extent, where there was a random elelement to the date you did, and the zombie's were a threat. They keep that, but still give you power and choice within a modern gameplay mechanic (free aim).