I had actually never even considered that people think “go into the trades” means “work as laborer #17 for your entire career.” The goal, obviously, would be to work as an independent business owner, eventually. That’s the attraction.
The trades are unique in that you have to be out on site, there isn’t all *that* much your boss provides that you can’t, and there’s more work to do than there are people to do it, so it’s possible to drum up business on your own.
Nobody is imagining their kids on a job site making $67k year in year 17 when they give this advice. They’re picturing an independent contractor.
People of high ability who enter a trade specifically with the goal of becoming business owners frequently make a large amount of money.
This has nothing all to do with the "average wage" of a employee plumber.
These types often have some quirk that prevents office employment, such as being dyslexic, having minor criminal convictions, explosive tempers, etc.