It's due to disease. You can consume a seemingly healthy corpse and end up with a prion disease.
And they tend to have cultures that are tribal and insular which has two main effects: prevents initial infection, and prevents spread of infection from one doomed tribe to another. Cultures that aren't as insular learn fairly quickly not to eat dead bodies because of disease.
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I mean, look at the french. We've been about this business of removing cemeteries for ages.