"Son, it needs doing." End of story for a farm kid, at least in my experience...
I think of it as one of those "build the pyramids" jobs, but I think that you start in one corner, and rock, by rock you make your way through the field. I'm sure that the rock picking happened many times over many years as rain, and freezing / thawing cycles brought more rocks up. I worked on a hay farm in Michigan that was still rock picking its fields after over a hundred years of cultivation. (And rock picking)
Still, I think the rock piles are amazing testaments to the labor.
All the best,
Peter