Did you ever see an animal hunting a fellow animal for food? Man is a social animal and it needs to hunt for so many things apart from food.
Apart from that, in our eco system, as per the rate of procreation of livestock, they can overrun the whole planet within a few years with no vegetation left to eat for them.
Man is a part of this eco system that helps the livestock numbers in check.
If you doubt my answer, you have many scientists who will underscore the answer above.
