Quite the contrary. Welders, carpenters, or heavy equipment operators in my
country enjoy a decent pay, yet there is a shortage. For some weird reasons
those who studied so sought-after political studies or literature can't be
bothered with a job that consists of physical work and doesn't require
a degree.
In the UK, plumbers and electricians can make more than developers do.
It depends where they work - London and the SE pay a lot better than the run down cities in the North - and how good their network is.
I used to know someone with a well-paid City job who retrained as a plumber. He earns almost as much as he did and is considerably less stressed. The only downside is occasional snobbery.
Say you have a small factory ,employing 50 older welders doing a risky job at relatively little pay.
Say you need 10 more welders - but those guys really don't like risk so they would need a big increase in the salary, maybe 50%-100%. But you know that than the word will get out and the older workers will demand similar salaries.
So it's a big jump in total labor-costs per work-unit. And let's not forget - revenue per employee can be quite limited in many places. So maybe paying low is the better state for the business.