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.