There's been a sharp decline in people with practical skills over the course of the past decade. People go to college and university and come away with qualifications in the arts and sciences. The general level of human knowledge has never been higher, but practical skill has probably never been less available. If you're prepared to get your hands dirty, you can make serious money as a plumber.
This all comes down to math. The fewer plumbers there are, the more they're in demand. The more they're in demand, the more they can charge, because they have a captive market. There has never been more demand for plumbers. And because you can charge a high rate for your services, you'll make money quickly and be able to hire another plumber to work with you. Progress from there to a team of plumbers with offices, and there's no limit to how much money you could make.