Because agriculture alone cannot support the world population. Millions of people rely on meat. They would starve without it. And the species of animals we eat are only alive because we protect them. If we stop eating them, they will all go extinct without our protection. Throwing livestock into the wild would simply give them a slower, more painful death. Also, there are necessary proteins and nutrients humans need from meat. Not everyone can afford alternatives, and there aren't enough of them to go around anyway. And remember that PETA completely exaggerates the pain pain the animals go through and blow it all out of proportion. You don't need to look very hard to see that their campaigns are riddled with logical fallacies and incorrect facts and statistics.