That's a terribly simplistic analysis. First, humans don't digest plants in the same way and won't get the same calories/nutrition out of the plants as a chicke

That's a terribly simplistic analysis. First, humans don't digest plants in the same way and won't get the same calories/nutrition out of the plants as a chicken/whatever. Second, feed for animals is significantly cheaper than the plants humans like to eat and what the stores sell them for. Sure, if you could live eating chicken feed then it would certainly be cheaper to eat that instead of chicken for caloric intake. But to optimize for dollar nutritional value in food humans actually eat, it's pretty hard to beat the $5 chicken.