Depending on the type of chicken your talking about, it will eat a lot more food to get to the same weight. And when it does finally reach that weight it won't be the same type of carcass.
My CX's eat between 6-8lbs of feed each to reach slaughter weight, they also are on pasture which helps that number. At 8 weeks to a 5-6lb bird that's pretty good. For a layer type of chicken they eat more than that just to get to 20 weeks of age, and unless they are a very large breed there's not much meat. Certainly not comparable to a Cornish X.
Your comparing oranges to apples, it's not even close. You can eat any chicken but they aren't all created equal.