Perhaps because they are the ones that actually convert the plans and raw resources into a useful building? Plans for bathrooms and buildings can be easily pulled from a shelf of existing plans, tweaked a bit, and sent out, but building the actual structure can never be avoided.
Mission accomplished. I never stated anything about my knowledge of construction. However, for whatever little it’s worth, I do know more about it than most people I encounter, especially in a forum like HN.
