I've the feeling articles on why Americans can't be found to do [hard manual labor, whether harvesting crops in Alabama or wherever] are perennials. But I noted Italians can't be found to do the hard labor of making cheese, according to this Marginal Revolution post. So why?
It's not genetic: we know Italians did hard manual labor when they were immigrating to this country in the 1890's. We know WASPs did hard work back in the 1630's and 40's and we know our ancestors did hard work at other times. So why can't Alabama farmers find Americans to pick tomatoes instead of relying on immigrants?
I offer the solution; it's called the "missing drill sergeant". In my experience there were two things, and two things only, which could make me do hard physical labor: one was growing up with it; the other was a drill sergeant.
By growing up with it, I mean this: by growing to be a man on a dairy farm I incorporated ideas of what was hard and what had to be done, what would make me respected among my peers when I hired out. I also literally incorporated the muscles I needed to do hard work and the calluses I needed to avoid the pain.
The other way I learned to do nasty things was through my Army drill sergeants. I was constrained by the situation and forced to do things I'd rather not.
I'd say the same applies to our workforce: we don't have slavedrivers and drill sergeants in the modern economy. Those Americans who grew up to do the work have, if possible, made their escape, just as I escaped from the dairy farm. So we rely on people from elsewhere, whose frame of reference from growing up in a less developed country makes picking tomatoes or working on Italian dairy farms seem at least tolerable, considering the financial rewards.
[Updated with a couple links.]