For me, farming is an auto-catalytic process because things that farming causes to happen then turn around and make farming more necessary. So the more you farm, the more you need to farm.
The best example of this is population size. When you start farming, you become able to support a larger population. But once you have a larger population, then you need to farm in order to have all the food needed to support this larger population.
This can account for the great disparities because societies that have farming tend to develop other things that help them have wealth and power. I am talking here about things like writing and centralized government and technology (not to mention germs).