Depends on the job. If it's something anyone with a pulse can do, then there's no real skill or reason for great pay.
There is a reason: basic respect for the person doing the job. Asking someone to work two full-time jobs just to be able to split the rent on a single apartment and still need food stamps is dehumanizing.
It would be different if low wages weren't used to create additional profit at the top, but the average McDonalds worker could get twice the pay per hour, yet still the corporation and various franchise owners would only see a dip in profits. This is the important part: their bottom lines would still be profitable, only slightly less so.