Today most accepted scientific theories, Darwinian theory of evolution, assert that human being is merely another product of billion years of evolution. As the theory assert, human is not really considered special in the big scheme of things. Human is just another evolved animal species. Therefore other kind of reasoning for why major cultures forbid the practice of eating human being is required to explain this phenomena.
It is interesting to note that while Darwinian theory of evolution is widely believed to displace human from the proverbial high place, the theory itself actually capable of confirming some aspects of major religious teachings. For example, the Darwinian theory of evolution consider that the practice of eating human being as something that doesn’t fit with other treat of human being.
Human Beings Are Not a Good Source of Food
When foraging or hunting for food, a creature have to make a risk-benefit calculation. The benefit gained successful hunting, should not exceed the expected probability of losing one life. In case of hunting animals for food, the probability of losing one life lies in mistakes that result in failure of hunting attempt. In case of hunting fellow human being, the probability of losing one life is amplified by the obvious future attempt of victim kins to take revenge even if the hunt was a success.
The act of consuming human also poses a serious risk for the consumer to get a disease known as Creutzfeldt-Jakob disease. This disease is also known as “Mad Human Disease” which is probably a parody of “Mad Cow Disease”. This disease is deadly and currently incureable.
Human Beings Are Intelligent
Before taking the probability of kin revenge, one should also account on the difficulty of hunting a human being. Human being are more intelligent than other available preys. The more intelligent your preys are, the harder it is for the hunter to plan and make a trap to hunt the prey. Once a rabbit enter a simple bamboo trap, there is nothing it can do. Chimps on the other hand are more cunning. The also have prehensile limbs they could use to unlock the simple traps.
Human being are more intelligent than that. Experienced human beings are capable of detecting traps intended to catch them. In case they understood that the traps were intended to catch them, they are going to hide and wait until the hunter show themselves. Then the ambush begin. Even if a cannibal manage to catch an inexperienced human being for their BBQ, the victim kins may come and take revenge.
Human Beings Are Social Creatures
Human beings are social creatures because we are simply more efficient in number. While a solitary human being may survive the element, there is no way for a solitary human being to survive a pack of human beings. Therefore a society of human, capable of discouraging any acts that lead to diminishing their number are going to fare better. The society is going to do so by eliminating what it see as potential dangers.
Non-cannibalistic tribes are going to view cannibalistic tribes as potential dangers to their tribes. They are going to start an ambush to finish the cannibals as soon as they got clue that the cannibals practice cannibalism, especially if one of the victim belong to their tribe. This is why rebel leaders in the ancient China sometimes accuse their enemies as cannibals to ensure the success of their campaign. If you heard that some defeated emperor was practicing cannibalism, I would suggest to take it with a grain of salt.
Moreover as social creatures, we can have greater profit utilizing our kins in ways other than consuming their flesh. Why should we eat our brother if he can help us to catch a lot of fishes?
So Why We Observe the Rarity Cannibalistic Cultures?
First, the observed rarity could be explained by anthropic principle. Our likelihood to survive and be able to stand here as an observer would be greatly diminished if we live in a world where cannibals are everywhere. If most people are cannibals, I am not going to be here and write this article and you wont be there to read this article. You might be stabbed to death and turned into barbecue as you read this very paragraph by your own brother or sister. No one is going to invent the computer you are using at the moment if everyone is busy stabbing each other back for food.
Second, the observed rarity could also be explained by Darwinian principle. By not allowing cannibalism, a society can save itself a lot of trouble. Non-cannibalistic society make it possible for different people to stick together and develop more effective weaponry. Making a bronze spear and shield for example, require ore-miners, coal-miners, traders, weapon-smiths to work together. It is impossible to make bronze spears if the ore-miners have to worry that the traders are going to turn him into his lunch the next time he visit. Over time the cannibals and their primitive weaponry and organizational skill will simply unable to compete against the more advanced non-cannibalistic society.