why are some animals allowed to have rights, while others allowed to be butchered and eaten?

The line is generally a combination of social, practical, and culinary. That is, if it’s not a companion animal, it’s not endangered, it is customarily raised as livestock and it is tasty those are all evidence it probably goes in the latter category. So chicken = food, whooping crane = not food because endangered, german shepherd = not food because companion, blue ringed octopus = not food because taste bad.