your heart can be blown to bits but you can still move around and be conscious. a good head shot will kill instantly, but it is very hard to hit the head of a moving target, far away while under duress. that is why you aim for center mass. it is the biggest part of the body and it will stop the shooter in most cases.
Anyone who's ever been deer hunting can tell the stories--heart-shot deer running 20 yards.
Same with woodchucks.