Plumbers and electricians don’t go to school for multiple years to learn a trade, they apprentice - learning on the job, until they get to the point where they propel themselves forward.
On a few educational systems, you have dedicated curriculums for technicians and artisans/craftmen, with theory in classes, practice in labs and internship on the job.
Even if learning on the job is a big part (roughly half of the training), it's not purely that.
The key (which I think you're making) is that once you're an apprentice you learn by doing and get paid.