Much of what we feed to animals is high-quality nutrients that can be eaten by humans instead:


Because of some ingredients being "inedible by humans" (stalks, leaves, oilcakes), or not as palatable as other options we have (e.g., wheat bran, potato peelings) - the animal agriculture considered as the helper to different segments of agriculture and food industry.

The animal feed industry productively utilizes the co-products associated with the refining of oilseeds into higher value food materials. 

Real reason- there's no cost-effective way to make it palatable and/or nutritious to humans.