Because we’ve passed peak agricultural land. The land committed to growing crops and pasture used for grazing livestock has peaked. The global population, however, continues to increase. There are methods for maximizing yields from farm land, and we haven’t exhausted those, but there’s only so much food that can be produced on a hectare of land. We also have to deal with top soil depletion, the risks of monoculture, the effects climate change could have on crop yields, and many other problems.