Cows are extremely inefficient. How can you say they do an outstanding job? Outstandingly high emission of Methane belching?
When Michelangelo painted the Sistine Chapel, it took him forever to paint a small surface. Any old housepainter could have been more efficient.
The point is that grass is inedible and grows all over the place, and cows turn this abundant and useless resource into wonderful steaks.