Two possibilities:
a) The unskilled will choose to clean airports anyway. They can't get a job as a doctor and they want to buy toys (TV, smartphone, holiday, etc). Equivalent salary does not mean that jobs are just as accessible - academic positions tend towards min wage, but the working classes aren't scrambling for those because they are locked out via credentials or lack of education in general.
b) If clean airports require exploiting the poor (basically, relying on the fact that people will starve and die or suffer mental illness if they don't do it), then maybe we shouldn't have clean airports.
I think that part of what makes some jobs so crap is the fact that employers know that their employees are reliant. Senior management jobs are cushy precisely because people have FU money and can just leave. This should be expanded.