This supplies a vast, vast minority of the calories in developed countries. CAFOs, or factory farms, tend to supply animal products to developed countries, and those animals aren’t being sustained on the grasslands. So your point doesn’t really justify the ubiquity of animal consumption.
The GP's point applies even more to factory farms than grazing animals because animal feed can be transported across the globe. It's the agricultural version of specialization.
The vast majority of feed used in factory farms is inedible to humans. The primary ingredient is often the waste product from bioethanol production and distillation [1]. The edible varieties of corn, barley, oats, and alfalfa used in feed aren't that popular with people [2], are mostly produced on land that would otherwise be unproductive, and are mostly used for newborns or in the final stage before slaughter to make the meat fattier and more palatable.
[2] For example, even though barley is a core ingredient to much of the alcohol industry, animal feed still accounts for two thirds of barley consumption. Most of the rest of the barley ends up in animal feed in the form of distillers grains.
