This question has been on my mind and I'm finally answering it? Hm.
I think it's because - you know what's the difference between humans and animals? - humans can voice out their fear and it's undoubtedly easier to relate to your same species. Do I need to elaborate?
For example, A is going to be killed for food and A knows about it (let's say it's a village setting with a system in place to decide who's the upcoming sacrifice), A will most likely feel fear and confide in their friend/relative about it. After all, survival is in our instincts. Then, A's relative may sympathize and relate to A, thinking "How if it's me next?" A's family may pity him or even oppose the practice itself for various possible reasons e.g. out of fear, compassion, logic, etc.
Killing and eating humans can be dreadful because you think it may be you the next time. It may be your family members. You may despise the practice when suddenly, somehow, it's someone close to you who's eaten. This is definitely not the case with animals. You may pity them slightly but they never beg for their life, their family would never take revenge on you, and importantly, you know you will never be in their position. That's why, I believe, you need extremely good reasons to resort to eating humans e.g. shipwrecked.
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