> most Americans don't want to do back-breaking manual labor for a living, and b) most Americans also want food to be cheap.

Combine these and you're right. Most Americans don't want to do back-breaking labor for a pittance. Grocery checkers (union) make $18.50/hour. Bus drivers (union) make $29/hour. Journeyman (union) electricians make $50/hour.

Try paying more than $15/hour for farm labor and you'll have more applicants than you know what to do with.