We have codes and laws about what wiring you are allowed to use, and independent inspections to ensure that things were done right. (even the best person makes a mistake). Adding a license doesn't really do much. I can teach a smart person how to write a house in a couple days (this includes looking up what size wire to use on the charts - the NEC is actually very readable once you figure out which section applies), from there on some experience will make you faster. Yet to become licensed requires years of experience, mostly to keep people out of the job.

Hypothetically we have code enforcement. In reality, the chance of getting caught on a non-obvious building defect is much lower than certain.

I see licensing as buying into the system and aligning interests. Suddenly, it's in a tradesperson's personal interest to follow regulations, as they otherwise risk their right to practice.

That said... as was mentioned upthread, there's a happy balance. If there's limited long term harm, it's probably not justifiable to require licensure.