Because the neutral wire and the panel, as well as the water piping in the building, are all electrically bonded together. The resistance between these objects is virtually zero, unlike your body which can be measured in the tens of thousand of ohms and up. The current is taking the path of least resistance through all of the metal instead of through your body. If however, the was no bonding between the neutral and ground, then it would act just as a hot wire now does and zap you.