You can be a trained retail worker in a few hours in most cases. Same with many farming jobs. Same with working the line at fast food
Retail workers have to deal with irate customers, develop product knowledge, and how to sell items.
Fast food workers need to learn how to work as a team, make the food at a consist quality, and juggle multiple orders at once.
As for farm workers, if they are working with animals: how to read the animals for their behavior, treat minor injuries of animals, and how to manage animals.
I will agree that all labor deserve a living wage.