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One of the biggest reasons is because cattle growers (especially in North America) feed their cows corn instead of grass or other things that cows actually evolved to eat. They do this because long standing US government subsidies on corn production mean that it can be sold for less than the cost of production; the farms are literally paid to grow it. This is also why high fructose corn syrup is in everything you eat. The corn makes the cattle sick, so the farms pump them full of antibiotics, because that's cheaper than just feeding them properly.