Becuase, this is like saying that the lungs and mouth should only be used to breathe and not to sing, or that the lips should only be used to taste and eat and not to kiss, or that the hands should only be used for holding food and not for dance or art or touching–a behavior shared with other primates. Or that our language should only and always convey higher level language because that is one trait that we do not share with other primates. Becuase you are arguing that the absolute standard for ALL behavior should be what is actually only a partial aspect of our behavior.
Let’s use that image of a lung to breathe and turn it upside down, borrowing ideas heavily from
Lost in the Cosmos and Peter Kreeft.
Most animals have lungs. Lungs are used to breathe. Some if not all animals use them with make sounds. Fewer imitate speech, but only one animal, the human animal, speaks with what can legitimately be called language, that is, as a triadic creature in triadic conversation rather than as merely a very complex series of dyadic reactions. People in conversation are not making dancings like bumblebees to direct another bumblebee towards a flower or apes who have learned to place their hands certain way to get a treat as they are also cognizant of themselves as being selves. Long-winded, said little, but a necessary foundation for the following observation: People are quite unlike animals.
If we are going to express how people are unlike animals, it cannot substantially have to do with how we use our lungs, or even the outward symptoms of making sounds, but the manner in which we talk, and not even merely that. We must concentrate on the uniquely human sentience which is cognizant of the self to really get to the core of what makes people tick. We must ground our understanding of humanity in this very real abstraction as not a matter of breathing or baying, but as speaking.
Just so, it is what makes us different from beasts which must ground any viable moral system. It is useless to concentrate the parts of our being which we share with animals, that middle part of the Venn diagram, because that is not what people are. More accurately,
what people share with animals is at best only peripherally what people are, a footnote to more than a chapter of our being.
Moral systems, after all, can only be used by sentient beings; to our knowledge, only humans. Is a broken vending machine or a naughty dog immoral? No. You does not admonish it for misbehavior, only malfunction. You don’t tell it to go to confession. You kick it.
As far as moral systems are useful, it is not in the traits we share with animals. It is in what makes us uniquely persons. (Incidentally, all of these qualities stem from the idea of the self.)\
Which only bolsters the most basic claim I’ve made:
Morality — rules governing human behavior — should have a foundation made of uniquely human qualities.
Substitute “human” for “sentient” as you will.