Can someone please explain to me why plumbers do not understand how flow is a consequence of pressure & bore?
All NVQ-qualified plumbers understand (or once understood!) these ideas, which are part of the Level 1 one curriculum, e.g.:
https://bpec.org.uk/wp-content/uplo...2-and-L3-Plumbing-and-Heating-Diploma-v23.pdf
One problem is that not everybody who does plumbing is a qualified plumber. Also, customers use 'water pressure' in a colloquial, not a technical sense. When lay-persons talk about water pressure they are usually referring to the properties of the jet formed at outlets and how the jet and flow respond to small changes in the control rather than what's going on inside the pipework.