You mean, nowadays you can't get documented workers to break their back on farms, under deplorable working conditions, for a tiny paycheck and no benefits. FTFY.
As a farm owner, I have to respectfully disagree. Check out the H2-A program. Though many labor contractors choose to not use this program, when they do, one requirement is to advertise the job to US citizens first. Typically, they can fill about 10% of the applications, and then a small fraction of those will actually complete the job.
Good wages and benefits don't alleviate the work involved with many farm labor jobs (try picking avocados commercially, or hand weeding a field for a day sometime). For better or worse, Americans are not physically capable of doing the work that the Mexican laborers are doing. We used to be able to, but life has gotten too easy for us and we don't have the same fortitude.
That is not meant as a slight to the American people (I am one also), it's just the same as not being able to drink the water in Mexico as a gringo. We don't do it, so we can't do it.
If we change our attitude toward work, there is certainly nothing stopping us from regaining this ability, but you fool yourself if you think you can do it now.