In 1900 it took eight (8) agricultural workers to produce enough food for eleven (11) people. By 1980 the ratio was one (1) to one-thousand (1,000). The onset of the oil age produced great advantages to a mechanized farmer. It also provided cheaper fertilizers, pesticides, and herbicides that increased productivity. This, combined with the 'dust bowl' era of Western drought, prompted a change from an agrarian to a manufacturing economy. The bottom line is that workers were needed in the city more than they were needed on the farm so, essentially, they followed the jobs.
I suspect that one day, when oil is no longer a dependable energy source, this trend will reverse itself somewhat.