Instructor screwups have sadly led to a number of student deaths when instructors acted in contradiction of their training. It happens, but not all that often. What do you think would happen if instruction were instead led by people who were not trained in those things and were likely to do those forbidden things because they thought they would be good ideas? The instructors who made those fatal mistakes would be sued, and fortunately they would have insurance. Would Uncle Bob have that insurance?It doesn't really seem work great for driving either. AFAIK, the US has double or triple the amount of fatal traffic accidents per capita than European countries that require people to take lessons from an actual driving instructor.
Instructors might put too much lead in you pocket and maybe are not that great a lot of the time... but I'm not sure a random dad or uncle would to a better job. A PADI instructor has at least a 100 dives, uncle Bob might have 8 dives when he starts teaching.
In my 19 years on ScubaBoard, I have seen countless threads in which people praise activities that they feel should be part of scuba instruction that would be open and shut lawsuits in case of a fatality. In more than a half century of instruction, people have learned things that Uncle Bob either doesn't know or doesn't believe.