Also important is the waste they produce. Cattle alone produce many times more waste than humans. Feedlots and factory farms concentrate large numbers of animals. It's not cost effective for them to roam free. As a result there are concentrated piles of manure.
Water use is also an issue.
Half the water used in the U.S. is for meat production. More than half of all feed is grown on irrigated land. Enormous amounts of land and water are used. It takes less to grow grain. It takes 16 pounds of grain to equal one pound of feedlot beef.