I think there's a pretty reasonable argument that 1) conditions for livestock are actually much better than those endured by animals in nature, and 2) living to eventually be slaughtered is better than never having lived at all.
There are many reasons to why we wouldn't eat certain animals. Cats hunt rodents, dogs are protective, horses provide transportation. Cows don't do any of those things well, and they also taste wonderful.
If #2 justified animal agriculture it could also be used to justify human slavery, provided you intentionally bred humans for that purpose.
And regarding my second point somehow justifying human slavery: that's only true if your moral system somehow makes humans and animals out to be equivalent. But we don't even make different classes of humans morally equivalent to each other, so why would we make animals morally equivalent to humans?