It hasn't. Livestock farming started about 10,000 years ago (give or take 1000 years), although this keeps getting refined with DNA studies and I'm not sure what the consensus is. But that also doesn't mean it's necessary in the modern era with modern agricultural practices.
It doesn't change your claim that it's been something we've been doing for thousands of years, but like I said- it isn't necessary in the modern era with modern agricultural practices.