Pigs are the ultimate recyclers, and we'd really not want to eat their inputs in much of the world. Have some organic waste? Well, feed it to the pigs!

As an aside, the Coptic christians in Cairo stopped collecting trash because the government slaughtered all their pigs, leading to a huge garbage crisis in the city.

Cows and sheep graze on marginal grassland that is unsuited to agriculture. Most countries aren't like India and suited to vegetarianism. Tibet would basically not exist without meat (and Tibetan Buddhist are specifically not vegetarians accordingly). The Inuits could never make a go at it in the arctic as vegetarians.