I think that in this case, the ick factor is because evolved traits can only work with relatively simple patterns.

My guess is for many of us, our gut says "looks like a human therefore is human"; if you try to tell gut instinct it's fine because there's no brain, you're gut's response is "Brain and brain! What is brain?"

My gut seems to care more about dynamic behaviour than static appearance, but for what it's worth — and despite being able to understand the premise of @echelon's suggestion without being upset by it — even I find images of a real, natural, human birth defect where the brain is missing, to be horrifying (content warning: do not google "anencephaly" unless you're strong stomached).