Because its "natural" and complies with the laws of nature.
I gave milk to your children. I gave milk to your ancestors. Why are you killing us?
"So God created man in His image, in the image of God created him; male and female created He them. And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth; AND SUBDUE IT; AND HAVE DOMINION OVER THE FISH OF THE SEA, AND OVER THE FOUL OF THE AIR, AND OVER EVERY LIVING THING THAT MOVETH UPON THE FACE OF THE EARTH." -- Genesis 1:27-28
Suggestion: PETA could have all its members and supporters Identify as being Bovine.........just a thought.