[lupusreal]

Two possible reasons why humans don't eat humans (except when we do):

We're hardwired to have empathy for our own kind, e.g. the "selfish gene" theory, stronger for kin than for strangers but nonetheless strong enough to create an almost universal taboo against eating people. Note however that many other animals do seem to be wired to eat some of their own children: https://en.wikipedia.org/wiki/Filial_cannibalism

"The most dangerous game." People are very resourceful, and furthermore have friends and family who hold grudges. Eating people is a bad strategy because people who make a habit of it tend to get killed for it sooner or later. Hunting nearly anything else is safer than hunting other humans.

[marcosdumay]

Just to say, that second one also explains why almost no other animal likes to eat people either.