I take no position on why these jobs are unfilled by Americans. But trying to claim these jobs are stolen or taken by undocumented workers (as implied by the comment to which I originally responded) is just wrong. If I assume you are correct (and it is in fact a quite plausible theory), I would allege the jobs are being stolen from American workers by the employers. Certainly the employers are relatively more profitable as a result of their shenanigans, if you are correct.