In India after the mother's milk, people drink cow's milk. That is why they call it Go Mata or Mother cow. This relationship has been since the time unknown. As they use cow's milk to nourish themselves they treat her as a mother. They give a lot of respect to the cow.
Having said that, it is obvious that because of motherly respect they do not eat her meat.
Apart from above killing of cow, goat, sheep or any other living being for any reason inluding meat is forbidden in oldest eternal divine book of hindus, Vedah.