The land in many of these places *can* support growing crops, with proper irrigation techniques etc... However, these places most of the time lack electricity, stable governments, water, money etc... to implement all this.
In countries like Africa, where people are starving because of lack of food, we ship in rice from other countries. Surely if the land cannot support the people living there, then the people living there should stop creating more people.
Would you still be saying the same thing about the human population when there are 10, 20, 30 billion people on the planet?