How much effort does it take to grow a bushel of corn? You have to plow the field, plant, maybe water if your area is dry, weed, and harvest. After a year's tending, you might get 150 bushels on an acre of land.
How much effort does it take to grow a cow? First, you have to grow maybe 500 bushels of corn to feed to the cow. Then you need to come up with around 8000 gallons of water to feed to the cow. Then, you are faced with the task of tending to all the needs of your growing cow until it is large enough to kill for food.
Why go to all the extra effort when it's not necessary?
Come on people, let's get lazy and eat plants.