I'd challenge the presumption that we can't hire citizens at $15/hr to do the jobs I'm thinking of.
I do agree that there are jobs that aren't getting filled even at today's higher wages, but that's not what I'm talking about. What I'm referencing is the kind of jobs I (we?) had as kids - de-tassling corn, delivering newspapers, working at fast food joints or as a bus boy at a locally-owned restaurant. Now, these jobs may pay $15/hr, but the expectations/requirements are much higher as each position costs the business much more.
On un-filled jobs, my personal believe aligns with other comments in this thread - we're paying people too much to not work that the incentive just isn't there. If it was starve vs work a $15/hr job, those jobs would be getting filled.