Absent a belief in a God who has designated humans as a special creation, there are really only two arguments for not eating other humans. Neither of those arguments apply to any other creature that we have so far encountered. The first, which others have alluded to, is social contract, "I agree not to kill and eat other humans so that other humans will agree to not kill and eat me." The second you refer to in a manner, but seem to overlook its significance. There are proven health problems from eating other humans. As a matter of fact there are several diseases which seem to enter a society as a result of cannibalism, but which do not seem to be limited to the cannibals (there has not been a whole lot of in depth research on the health impact of cannibalism, largely because there are so few cannibals).