"They do the jobs Americans won't do" has become a self-fulfilling prophecy. What it really means is that Americans won't do these jobs for Mexican or Guatemalan wages, since illegals can be paid under the table at a dollar or two an hour.
When I was a kid, there wasn't a single Mexican or Central American "migrant worker" in my town. Yet as I recall, somehow the berries and vegetables in the local farms were picked, the landscaping got done, and the trash cans were emptied.
Amazing isn't it?