So many things are caught. At best there is a lack of QA on the Chinese side, but it's definitely worse than that. They have no qualms sending you known-bad items, and just see it as "maybe the customer won't notice" and worth a try. It's definitely a giant pain in the ass, and adds a ton of expense and friction to the process. Lots of stuff you simply cannot source from any other country though - even if you do, the supply chain usually traces back to China anyways so all you're doing is adding a middleman layer to the problem.
It's extremely important to set very explicit and strict quality parameters and specifications prior to any deal you do with a vendor. Even then you will miss things that will later be argued about. The more you can specify the better, otherwise it will be seen as negotiable/changeable.
A lot of folks used to living in a high trust society get really taking advantage by this. Any vendor sourcing from China and not implementing an extreme level of QA to the process is being negligent, and I assume that's quite a lot if not the vast majority.