Don't know for sure, but I rather doubt bank tellers make much either---they just need to be able to pass a drug screen and background check, be reasonably intelligent....all the other jobs you mention require training of some sort, at the very least, extensive o.j.t. There's damn little to be done at a Walmart by non-management folks which needs more than a few days of training....if ya ever take a close look at the check-out people, it seems obvious
You really ought to go check with some banks about employment. They have been limiting tellers to less than 40 hours a week for years to avoid having to pay benefits. Part time workers in there have saved the banks tons of money in employee benefits. It's not a wally world invention you know.
As far as factory work goes, come on. Are you saying it's hard to teach someone to install a tire on a car? It's hard to train a person to solder a wire into a light fixture much less just screw parts together the same way day after day after day?!?!?!

Not really any different than requiring a cashier to be familiar with the relative prices of store inventory, handle cash and maintain positive control over intake and out flow of same.