This is probably key. If GP's comment is accurate, I suspect it has more to do with "small business owner" than "tradesman". Of course they may just have a inaccurate view of real net, also. I know a few guys who would brag about "I made X last year" as a self-employed sub, but on a little interrogation it became clear they were very handwavy between company gross revenue and "I made".
Plumbers: $60k https://www.salaryexpert.com/salary/job/plumber/united-state... https://www.salary.com/research/salary/alternate/licensed-pl...
Now those aren't self employed, but
"What Kind of Money Can a Self-Employed Master Plumber Make Annually?"
Suggest maybe 20% more for self employed
https://bizfluent.com/info-12088771-kind-money-can-selfemplo...
"The salary for a master plumber with more than 20 years of experience ranges from $43,790 to $74,700 a year"
Do you have any data rather than your self-selecting annecdote?