I think your question is going to have a different answer for everyone, but for me: A) The average human fears and understands death even at several removes, a

I think your question is going to have a different answer for everyone, but for me: A) The average human fears and understands death even at several removes, and in particular would understand they were being raised for slaughter. I don't believe this to be the case for most farm animals. I don't believe chickens suffer from expectation in the same way humans would, and I wouldn't eat e.g. great apes because of the possibility that they might. B) It is unacceptable to allow the killing of humans and the desecration and consumption of human remains except in some extreme cases, because doing so would harm our social fabric and make still-living humans feel awful. This protects even humans who wouldn't be able to understand abstract future harms. C) Also, it feels wrong, and killing a chicken doesn't. I know this answer is likely dissatisfying to you because you feel that killing a chicken is also wrong, but we're all making choices by gut feeling to some extent.