You, Sir, seem to be uncommonly blessed with really good dive shops!
On a smaller level, the whole not selling parts (see how I brought us back on topic?) Feeds into the the FUD of "it's life support equipment, leave it to professionals," and "it's life support equipment, if you tamper with it you'll die." If they made parts and manuals readily available and laid off the FUD, then the average diver wouldn't treat the gear as a magic talisman keeping them safe under water.... Then they might learn about the gear, which could lead to understanding. That would lead to them learning that most new models are the same 60s and 70s tech with a cosmetic change or two.... And we can't have that. Gear has had incremental improvements over the decades, but no real game changers... And if the average diver understood that, sales would drop drastically.
Respectfully,
James