Humans don't have the cellulase enzyme necessary to digest cellulose, the primary component of vegetables. Cows and other grazing animals do, which is why they

Humans don't have the cellulase enzyme necessary to digest cellulose, the primary component of vegetables. Cows and other grazing animals do, which is why they can efficiently use plants to gain energy and mass. Humans eat meat because we can not use the majority a plant's mass for energy or mass production. Some day maybe there will be a cellulase supplement or probiotic supplement that allows efficient veganism, but we're not there yet.