Why do not employer simply let people learn about their jobs as they perform them?

Why do not employer simply let people learn about their jobs as they perform them?
Some people are faster learners that others. The employer needs someone who can be immediately productive. In some jobs there is also expensive equipment involved that the employer won't want an inexperienced person using and possible breaking it. There are jobs that can be learned "on the job" with the right person and some that can't. It can also depend on the experience the employer has had with "trainees".