The corn consumed by cattle includes the corn stalks which are more or less the same as consuming grass. Corn is after all in the grass family. The actual kernels of corn increase the calories versus grass, but it’s not like cows are just eating corn off the cob all day long. They need to consume tons of roughage to help with their digestion. I think a good argument is that converting grain and grass to beef is inefficient - if you could have consumed the grain yourself, but the nice thing about cows is that they convert very low calorie foods into high calorie meat that we can consume. If we had to eat what cows do, we’d be chewing all day.