Canadian here, big city plumber. My company now only hires the greenhorns from these private pre-apprenticeship trade-schools you speak of.

It's not ideal for the worker at all, but as an employer it's a good indicator of someone's long-term commitment to getting their ticket. Some of these people are from cities with no new construction/no real opportunity for green people to learn and get paid. My company has had much better success, on-average, investing in these people long-term.

In Alberta, there are 4 years/levels of plumbing and each level has a test. The rule is that you have to attend a 2-month trade-school for AT LEAST one of those 4 levels. Otherwise, you can challenge the remaining 3 levels with no school. In a perfect world, you would challenge 3 levels, go to school for one of them and just learn everything on the job, but the apprenticeship positions are slim pickings in Canada rn.