The problem would be minimum wage and insurance requirements for employing citizens. There are plenty of citizens that would work those jobs but nobody would hire them because they cost too much. What you are arguing for is to continue allowing people to come here so employers can pay them less than a citizen is legally required to be paid. Once they become legal employers no longer want to employ them for the same reason they don't want to hire citizens.
US citizens by and large don't want to go work in tobacco fields for $15/hr, in a state with $7/hr min wage. But mexican workers coming over legally, getting the work visas and all that... will.
