Zombies are the soldiers/police of the world, conflicts happen anywhere all the time especially of late.
The world as this in built desire to destruct and belittle any other to gain the upper hand, add to this the belief of 'the dark side' of life, what i mean is using scare tactics of old say via religious indoctrination (ask for help forgiveness or be sent to hell), the modern equivalent would be using a movie or ancient beliefs with a modern spin on them (keeping it hip).
Most youngsters have grown up with game stations that kill anything moving (with your help), all this does is further brainwash and ingrain the beliefs of old.
The constant barrage of good V evil via TV in what ever form as made humans so caught up in the whole life as two passages you choose which one.
The one is a normal day to day life how you make it, the other is a fantasy mirrored existence walking the shadows looking in on reality, simply because you fear normality.