https://www.sciencedirect.com/science/article/abs/pii/S2211912416300013

86% of the global livestock feed intake in dry matter consists of feed materials that are not currently edible for humans

But it also makes an important contribution to food security through the provision of high-quality protein and a variety of micronutrients – e.g. vitamin A, vitamin B-12, riboflavin, calcium, iron and zinc – that can be locally difficult to obtain in adequate quantities from plant-source foods alone

Just because they feed corn doesn't mean it's edible to humans, a lot of the corn grown is left to dry on the plant and then harvested. We do this so we don't end up with another famine. Not saying corn is what we should be growing for that, but it's a very easy and hardy plant.