Well, a lot of chickens are developed bc humans put various breeds together with a goal in mind (winter hardiness, egg laying, more meat in a shorter amount of time, etc). Chickens are a great nutritional resource. So, the BJG was a naturally developed breed from combining other breeds and historically, they were bred to make a larger table bird, like a turkey. But then the broad breasted turkeys were developed, so BJG never really took over the Thanksgiving market. You can't really knock human's ingenuity - different chickens serve different purposes. And, if a person was to rely on their flock for food, they would likely breed from the better egg layers, or from the larger birds...to continue to get more eggs or larger carcasses.