Some of those are only at the discretion of the employer/commissioner. For instance, your teacher and engineer examples. When the government is the employer, th

Some of those are only at the discretion of the employer/commissioner. For instance, your teacher and engineer examples. When the government is the employer, they have decided they want to hire only within the pool of those who have certain credentials. If you want to hire someone for private works, you are free to make your own choices and suffer the consequences if you choose wrongly. I think it is fair to say that is not artificial. That is market choice. Some of your other examples, like lawyers and dentists, however do seem artificial. There is no legal option to hire someone without credentials (at least in the jurisdictions I am close to), even when the buyer understands the risks and poses no external risk to others. I don't know about this heavy machinery thing. As a farmer, all I need to drive my 30,000 lbs. (when dry) combine down the road is a standard automobile license, and nothing at all once I'm off the public roads. Nobody thinks twice about that. Even more curiously, I do not need any credentials to produce food that people willingly stick into their mouth! You'd think that would be orders of magnitude more worrisome than many of the other careers you pointed out – especially given the difficulty of tracing food origins as a consumer – yet nobody bats an eye.