Many states require a license for hair dressers, and DOGS!
If you don’t think electricians and plumbers, etc. are high-knowledge jobs, i’d guess you don’t have a great understanding of the professions.
I understand HVAC, plumbing, and electrician just fine.
These are not high concept jobs. They are “follow the print” types of jobs. The design of these projects, the materials and methods are specified in the engineering drawings for the specific job element. They need to know enough to know that the materials provided meet the specification and how to read the drawing to complete the task.