To be fair Korea has like 50 years of experience of doing the grunt work of animation for USA and Japan. It's even in old Simpsons jokes. Koreans got so good at it that they're now too expensive for most studios and Japan now uses such exotic places as Vietnam and Thailand for third world labor costs.
China? Well, the Chinese racial trait is mimicry and they do it very well. It does bother me that they try so hard to trick people into thinking their stuff is Japanese, even getting the voice actors. But they really don't know any other way of doing things. Mao nuked their history and culture so they have to look elsewhere.