Because we don't have the technology to get rid of shitty jobs yet. We can't make that technology come faster by eliminating incentives to perform shitty jobs.
If you're in a world where you don't _have_to_ work for a living; you can take your time meeting your basic sustenance needs with real food. Fast food is actually a bad deal, and most of the people eating it are doing so because they are under time-pressure of one sort or another.
