I think it is a cultural difference. China can make excellent products, it just takes constant supervision and massive product testing. And make sure that testing doesn't follow a pattern they can game.
Production managers make large amounts of money taking shortcuts, skipping production steps, substituting inferior materials, etc. Culturally, getting away with something is the same as doing it right. If I unload inferior goods on you, it's not my fault for doing it wrong, it's your fault for not catching me.
The Japanese culture does not operate like that.