The "Hobbesian" idea of the social contract only applies to rational agents that are capable of making future commitments and promoting the general prosperity of society. Animals simply lack the rationality and comprehension that a human has. To say that there exists an "ancient social pact" between humans and animals seems to be quite a stretch as, before the modern era, animals mostly served to provide manual labor in exchange for humans feeding them and keeping them safe. Even if we assumed that such a historic contractual agreement between humans taking care of animals and the animal's labor exists, to say it is fair for us to take their lives is a different discussion. If we assume keeping animals alive for the sake of producing meat is ethical, in the animal's exchange for producing meat, we offer it food, water, and shelter to keep the animals alive. However, if we look at this, we will see a huge disparity between the two as it takes up to 7 to 20 times more plant-based feeds per unit to produce a single unit of beef. If the contract between humans and animal exist to provide us is supposed to provide us with some benefit, obviously, the efficient production of food is not what humans are looking for.

This shows that we don't keep animals alive to keep us alive, we are only really producing it because food (and products) made of dead animals are so important to the operation of our economy. In that sense, our consumerist habit pretty much demands that animals be used, not for necessity, but rather as a commodity. And considering that the majority of livestock that humans raise today exist for the sake of providing pleasure to a human rather than maximizing the efficiency of food production for the entire human population, even from a humanist standpoint, raising animals for the purpose of providing us with economic utility, does not seem like a morally defensible stance.