>All moral systems may be arbitrary (a perspective that by the way is a part of philosophy), but some are useful, perhaps even necessary, for the well-functioning of society. Discussing how to make, extend, and enforce these moral systems seems likely relevant to any society made of independent agents, whether human or not.

There are physical structures in your brain that point to genetic transmission of moral behavior. This indicates that morality is a product of natural selection. Moral behavior exists because it helps us survive similar to how hunger helps us survive. So what is the point of creating a philosophy around Morality when we don't create such philosophical studies around hunger? Why is there a philosophy of morality and not a philosophy of hunger when both sets of behaviors are arbitrary and serve the purpose you stated: "but some are useful, perhaps even necessary, for the well-functioning of society. Discussing how to make, extend, and enforce these <hunger/energy seeking/sex-drive> systems seems likely relevant to any society made of independent agents, whether human or not."

Clearly humans do not act morally. Pure moral behavior does not aid in survival. Rather a blend altruistic behavior and self interested behavior aids in survival. How natural selection implements this behavior is arbitrary. For humans, natural selection has NOT chosen to implement survival behavior through a singular processor, in short we do not see our survival as a blend of altruistic and self interested behavior. IN fact we do not even behave with "survival" on the forefront of our minds. Rather our behavior is implemented by our brain using conflicting emotional goals. These goals aid in our survival and exist for the purpose of our survival and are awareness of this fact or lack thereof is not a requirement.

We are aware of what evil behavior is, and we feel the emotions related to temptation we also feel guilt... we are also aware of good behavior and we feel emotions that reward us for altruistic behavior but also punish us when we do too much good to our own detriment. Genetic tuning of our emotional response largely influence our behavior. An evil person feels little response in harming others and an altruistic person feels great reward in helping others.

Morality and law arises as a phenomenon of our neo cortex attempting to parametrize and formalize these emotions. It is a mechanical side effect of trying to make greater sense of a mechanism that only has a singular purpose of survival. Morality is just a made up categorization of behaviors for a specific system.

The question is why then do we spend so much time trying to formalize ethics, fairness and altruistic behavior but spend almost no time trying to formalize other survival instincts like hunger or lust?

The hypothesis is complex but it exists. The neocortex (our higher level consciousness) is said to have evolved later in the evolution of life as a sort of higher level layer of consciousness that has executive control over our overall actions. Instincts like hunger and sex drive are baser survival instincts required for all things to survive and evolved relatively early before the neocortex. Many living things with no higher level consciousness such as bacteria display primitive versions of these instincts.

As a result our neocortex has no trouble recognizing hunger instinct or sex drive as separate modules within the human brain as these modules remain very well separated as they were created in different times of our species evolution. Therefore we don't care to analyze these modules as they are easily recognized as base instincts.

Group behavior involving altruism on the other hand evolved at a later time with the neocortex. Altruism aids in survival but its' benefits are not immediate and as a result like the neocortex such behaviors take longer to evolve. As a result it is harder for us to separate and see that these emotions are the same basal instincts as hunger because these modules are so integrated with the neocortex as they evolved together. We make the mistake and assume that morality arises from logic when it is in fact just a set of emotions reacting to certain actions and behaviors.

Due to the fact that the morality module in our brain is a product of natural selection you can actually probe it to find internal illogical inconsistencies that are unresolved because resolving such conflicts do not aid in survival. See the trolley problem: https://www.wikiwand.com/en/Trolley_problem and various edits to it like the fat man or the fat villain. Arbitrary edits that change the circumstance but not the overall logical moral consequences have a huge influence on human response showing that our modal module is not internally consistent because there was no need for mother nature to make it fully consistent as it does not aid in our survival. When you examine moral conflicts try to self-reflect. You will find that like all people you try to make sense of why you "feel" something is wrong in once case and why it isn't in another case. This process is exactly as I described, an attempt to formalize the resulting output of the moral module of your brain, but you just don't realize that this is what you're doing.

If it is worth studying morality as a philosophy then it is worth studying hunger and sex-drive as a philosophy as well. Philosophy chooses morality as a zone of study because it does not fully strip away the emotions and biases involved with the human experience in the same way that logic and science does.