That's because humans take too long to grow; they're inefficient as livestock. 9 months to gestate a single baby that would take 16 to 18 years to get to adult size for harvest? The American South has already plumbed the depths of "humans as livestock." It was just missing modern, industrial equipment.
Humans are competing against: cows that get to adult size in 2 years and then you can milk or eat them, or pigs that give a whole litter of new pigs?
If humans grew to adult size in like a year, prior to learning about the health risks, humans would definitely be farming other humans for food.
People don't think in terms of species, they think in terms of relations. That's why some people feel worse for a dog in the pound than they do for the countless numbers facing genocides across the planet.