Because if everybody became Vegan, we wouldn't need the animals. In fact, they'd just get in the way and eat all the other crops. So.. we'd probably just get rid of them all. And then even more animals would go endangered or extinct.
Is THAT what you want? For pigs and cows to go extinct?
Maybe if Pandas tasted a bit better in a burger there'd be more of them around today.
Animals LIVE because of me. I allow them to have a whole (albeit short) life, because I eat them, otherwise they would never have existed.