Fundamentally eating meat is a behavior our societies want to preserve; setting exceptions gives some leeway and carve safe spaces, while also making the rule more powerful.
For instance settings pets as off limit make it clearer that non-pets are for consumption. Farmers can spend time and develop attachment to their cows, understand they're not inherently different from dogs or ferrets in intelligence or conscience, but we've made it clear it's ok if they end on our plates. Wherever the line is drawn (conscience ? cuteness ? % of DNA close to us ?) doesn't really matter I think, as long as the impact is not critical to our economy or society.
To go to darker places, it's the same kind of logic used centuries ago between christians and "pagans" ("they have no soul", it's ok to kill/enslave them)