I think people are mainly driver by the tabu against eating human meat rather than any kind of proper thinking about it, but the tabu itself probably came to be because people kept getting sick when they ate human meat but not when they ate other meats.

You see a lot of that kind of thing in other tabus, for example the ones against incest (inbreeding tends to produce offspring with health problems) or handling feces (because the bacteria in feces tend to cause disease much more than the bacteria in things like dirt).