Why do they try to discredit Joe the plumber by saying he has no license?

The man was approached and told of his vocation. His name is 'Sam' and if he was working as a plumber for a licensed man, and did not know his job, he would not be working long. Even a license does not make a person a pro. People with all sorts of certification, screw us all the time among them lawyers, doctors, but seldom hair-dressers,or any other trades person. A registration is all that's needed, and the market itself will weed out the incompetence. A certain amount of risk is necessary for progress, that is what brought us to the moon. We have a fine police force that are well trained, yet they can not keep you from crime, and are overwhelmed in solving theft and homicide cases. A bad plumbing repair or installation will be noticed by almost any idiot that hires them, just to save a 'buck'