Most of the animals are bred sick just to maximize production. The answer to your question is self regulation - the amount of animals we are producing is absolutely unnatural. It is not like millions of chickens would populate the world if we wouldn’t exist. They are artificially fabricated in factories.

Why should a cow bred for maximizing milk production should exist and what should they do, jump around in the fields? A normal cow from some decades ago could get older than 20, while they are nowadays bred to produce as much milk as possible and die within 5-6 years.