Because the cow might have died in a quicksand... the head must have remained outside but the rest of the body got buried in the quicksand...
if there is no quicksand nearby, the skull mighthave been brought to its current position by external agencies lik wind, animals, birds,etc
Actually cows doesn't found in deserts, Cows don't go anywhere near deserts. Its just represent of how harsh the desert is. Only skull of the cow is shown because of the movement of sands in desert by the wind, would shelter up the tinnier skeletons of the bodies. Its just a artistic way of show a cruel environment. Its just a sense of humor cows doesnt found in desert in actual.