Because most of our crops don't grow in tropic weather. Because the trade winds will cause an inversion between the Tropics and halfway to the equator, independent of the global temperature, meaning that there will be deserts north and south of the equator anyway. Because rising sea levels mean lost agreable land, which not necessarily gets replaced by the same amount of newly agreable land in the North (In the South, there is just ocean and no new land to be gained by moving climate zones closer to the Southpole). Because the amount of food we grow is a limiting factor to mankind. In fact, the total amount of land used to grow crops is slightly shrinking in most regions of the world, be it due to urban sprawl or re-forestation.