The parent lineages have been specifically bred for the past 40-50 years with one sole purpose in mind; creating the perfect hybrid broiler.  The "Cornish" and "Rock" look nothing like you'd see in a backyard.
Now, with that said, what's perfect for someone growing thousands of broilers in confinement, with a controlled climate, will not be the same bird small producers will seek.  Whereas having a broiler ready to go at 42 days of age is the priority for the industrial farmer, we in the backyard have to consider other things like mortality, disease resistance, vigor, forage ability, etc.
Some people raise very good Cornish crosses at home, othere really struggle with teh losses becasue the Cornish cross is not the ideal bird for a pastured (or free range) production.