I suspect you are correct. Plumbers and electricians are supposed to be licensed by the state; roofers, framers, drywallers, etc. don't need a license.
Of course, if they can now get a driver's license, then why not a plumber's license?
That doesn’t stop them from working FOR someone with a plumbers license or electrical license and doing the work. We live in a house that I know I was thrown up by an illegal alien building crew and the electric wiring is down right dangerous.