I've done a lot of that type of work myself. It isn't hard to learn how to do it right from books - and I have passed inspection reports to prove it. I've also seen those professionals do a terrible job - to the point inspectors admitted to hurrying my job because they knew the next would be a mess.
I believe you. But not everyone is an autodidact. Most people, for whom becoming an electrician or plumber is the best option among all viable careers, do not have the discipline, aptitude, and intelligence to learn the theoretical and practical knowledge of a trade completely on their own. And vice versa, people who would be able to pull this of, typically have options that are better-paying, higher-status, or less physically demanding.
