It's not just a matter of "paying a living wage", it has to be a wage high enough to lure people out of the city to do the job, into the unknown countryside.
That, and city kids will take time to adjust to the physically demanding work, so they won't be as productive at first, either, and trying to force things will result in even more injuries than normal (farm work is already some of the most dangerous out there).
So the farmers would end up stuck paying more for less, the workers would be stressed and unhappy, and everybody loses.
The migrant laborers who do this work year after year actually know what they are doing, and their labor is woefully undervalued.