A problem that makes this particularly bad is that many now don't pay at all until you manage to become qualified as a high-skill specialist. A lot of the low-end work has been automated, which has gutted the traditional pipeline where you start with low-skill jobs and move upwards. It's not just that low-skill welding jobs (say) don't pay well; they simply don't exist. Only high-skill welding work is in demand, and a beginning welder is very likely to be unemployed. He or she might try to get free work to improve, but even that is iffy. I'm not convinced the odds are actually better here than the (already not great) liberal-arts degree -> unpaid internship -> office job route. If you really want to DIY a trade without a college degree, I'd recommend learning PHP or web design before I'd recommend trying to break into welding. Or learn something about CNC machines and get a job servicing the machines that replaced those low-end trade jobs...
I looked into this at some point out of personal interest, and really the numbers don't bear it out. There are a lot of anecdotes about how we "need skilled tradesmen", but the pay and jobs just aren't there. When you take into account the lack of training opportunities, the high risk of injury, the generally short careers, and the unstable unemployment, the expected earnings from taking up a trade are much worse than just getting a college degree.