Sure, a series like Hannibal can make something as horrifying as cannibalism seem nonchalant. The reality is, there are many terrible things that could happen to your body if you only stuck to a diet of human flesh.
AsapSCIENCE created a video that detailed exactly what would happen to humans if they ate a diet based on a buffet of bodies.
For starters, you can run the risk of contracting any blood borne disease (hepatitis, ebola) the person you're chowing down on may be carrying, as well as infections like E. coli and other bacteria. Humans are also considered to be red meat, and about half of our calories are just straight fat tissue. Finally, we can run the risk of prion disease which can mutate normal proteins in the brain, causing neurodegeneration.
From a factual standpoint, the risks far outweigh the benefits.
While cannibalism is more common in the world of insects and some indigenous cultures, most mammals have a natural tendency to steer clear of the act. Probably for the best.