Because they are food. There are strict regulations on humanely raising food animals, and how they are to be slaughtered. Same thing with hunting for food - there are hunting seasons, and times that you cannot shoot a female deer for example as it's the time that they are caring for their fawns. We have a taboo about eating pets - yet other cultures do not. They eat horse meat, rabbits, guinea pigs (in Peru they are raised for food, not pets). It is not unnatural to eat meat - Nature created prey and predator, and a wolf that eats a deer is not cruel, just surviving. As long as those farm animals are properly slaughtered there is nothing wrong, unnatural, or unethical about slaughtering them for food.