I suppose if you advertise a job for $20/hr and a bunch of people show up and apply for the job, you're probably not going to start advertising the job for $40/hr instead.

And whether $20/hr is a "living wage" depends entirely on your circumstances. If you're a solo adult you can probably swing it. If you have 3 kids you will probably be on food stamps. Should Amazon pay people with kids more? Or only hire single people with no dependents?