In talking with a dietician and a beef specialist, they tell me that the DNA in living things determines what the tissue and other characteristics will be like in a creature. The creator made them this way! Actually beef and fish have some of each type of fat. The beef specialist commented that unsaturated fat is liquid at room temperature, so a cow would have trouble if its fat was mostly unsaturated. It would be very soft and squishy. I personally believe our Creator of the universe knew what He was doing and made them just right.