Because doing something just good enough is not even an option in Japanese culture. Do it right, do it the best you possibly can, strive for perfection. It is a reflection on you as a person and the collective culture.
The western corporate practice of planned obsolescence is not something that would have been considered in Japan when Canon, Sony and Nikon Et-all started making cameras, (and everything else japan makes.) That has change somewhat for some things as the world is taken over by Mega-corporations dependent on continued conspicuous consumption. A Toyota of today is not a Toyota of 1972.
These are my words from my understanding of the Japanese Culture and mindset that was forged from century’s of striving to be the best and do it right.
Why do Japanese companies dominate the camera market?
Because they produced the best, people recognized that and purchased their products, and because of the their cultural mindset they spent the profits on seeking to do it even better.