Knowledge of how to do X isn't the only relevant qualification for being licensed to do X. In fact, I'd argue it's not even the primary qualification. Something

Knowledge of how to do X isn't the only relevant qualification for being licensed to do X. In fact, I'd argue it's not even the primary qualification. Something else underlies it: the ability to be trusted to do X without causing damage or harm. Take an example from another licensed practice: operating a vehicle on public roads. Knowledge, skill, and being of age are enough to elicit an initial trust from your local authorities. But if you violate that trust by, say, willfully disregarding a red light, your license may be revoked, not because you forgot what red lights mean, or because you stopped being skilled at using your brakes, but because your holding of this license is now demonstrated to be a threat to public safety. The objective of a licensing system is to protect others from harm before it happens. The threat of losing a license is a good deterrent, and better, often, than the threat of criminal punishment, precisely because it's easier to invoke. (You called it "for arbitrary reasons", which is false, by the way.)