False equivalency. If you eat brain of prion-infested sheep, it will end with your death. With cannibalism it continues after members of your community eat you at your funeral, that allows for much wider spread of the disease. We don't practice cannibalism because of our own cultural norms, but that doesn't mean there aren't other good reasons for not doing it. (And question is whether out cultural norms against cannibalism didn't develop just to save us from this, after all there are speculations that it was kuru-like disease that caused extinction of Neanderthals[1])

[1]: doi:10.3390/pathogens2030472