Manual weeding is really not a job people want to do in the developed world. IMO it's the worst part of farming in that requires working at ground level all day and needs to be done every couple weeks. It will wipe you out physically. Typically people who have to do this for long and have any other job available to them will switch even for a pay cut, but farmers can't really offer much because food production is low-margin (and in the USA they are competing with farms that use herbicides).