There is more to it than that. You may be able to be trained in a few hours, but mastery and efficiency come with time. Someone who works in retail or farming can output more productivity in a shorter amount of time than an average person. It's not about how long it takes to train someone to start the job, it's the mastery you learn over time. As you do a job longer, your productivity increases, as you move deeper and develop more skill at the job.