Pasture land isn't really usable for anything else, and the level of destruction caused by converting land to monocrops far exceeds that of animal grazing.
A lot of pasture land, for example in Amazonia has actually been created by systematic deforestation, also in Europe, which was deforested a couple of centuries ago.
So a lot of that land could be used for reforesting. And if it would be so simple to use that land to raise cattle, why are there factory farms then?
It sounds cheaper to raise the cows on grass which is free food, unlike rations. Clearly the amount of meat produced that way is not sufficient for the demand.
