Some do. and we (as a species) still eat them.
They might not be speaking English, but they still have intelligence and a developed language.
Maybe it's the way of things.
Personally, I prefer to keep my daily meat consumption to animals that don't have very much to show in the way of intelligence. If an animal was actually able to rationalize out the words "please don't eat me," I think it'd be a cruel thing to eat them.
I think the absolute line would be that when a species is self-aware enough to ask "who am I and what is my place in this world?" it's time to stop eating them.
Until then, bring on the steak.
