There have been times and places in the past where it was simply not profitable for market-oriented farmers to pay for labor. This led to rather rapid shifts to smaller owner-operated farms, where family labor was highly motivated to work their own farm because they actually profited from their own labor.

Not terribly likely in the modern situation, for all the reasons mentioned elsewhere in the thread. I'd also like to toss out compulsory schooling as another reason for the decline in willingness to do farm labor. People get used to living and working in a certain way, and for kids who are used to being imprisoned indoors for 8 hours a day most of the year, the transition to working outside is just a bridge too far.