Why not a plumbing degree? Because it would confuse plumbers with heating engineers. A domestic plumber's main skill is to make waterproof joints, avoid airlocks and get a pipe across a building without causing excessive structural damage.
Heating engineers, however, have to design the whole system to specification from scratch. A physics A-level student could become an expert domestic plumber with a week's training and a few months' experience. They could give up three years' income plus fees to get a degree if they want but that does not make plumbing an appropriate degree subject.
Queen's University Belfast