Cows produce milk only so fast. You can't make them go any faster. With people, you can drive them faster. Cows require FDA approval. You have to breed cows.
People breed by themselves.
With people, you can drive them to the breaking point, give them drugs, drive them there again, and repeat until heart attack or burn out. You can't do this with cows. It would be considered cruelty to animals. If a cow gets sick you are really in trouble. If you give a cow drugs to make it better, you can't sell the milk or the meat.