From my understanding of the economics of the majority of US beef cattle, they are fed a diet composed largely of corn and soybeans, which grow on land that could otherwise produce varieties of the crop edible to humans. They are grown in CAFO (concentrated animal feeding operafion) lots and produce huge amounts of waste. This is in contrast to the more 'expensive' grass fed cattle that does indeed eat plant matter otherwise indigestible to humans. Our food system is designed to produce food in massive industrialized systems that trend heavily towards requiring massive inputs and producing massive waste outputs.