Lots of tradesmen enter because they enjoy the work and the challenge, and don't pay too much attention to business.
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.