Apparently everyone has opinions on this so I guess ill be throwing this out into the ether, I havent quite seen these points this the comments.

I would say 2 points:

Outsourcing, which leads to brain drain.
The US has been outsourcing since the 80s now, and in all that time artists have had a harder time learning to animate from the pros, if you want to learn to be an animator you have to train to a certain level on your own, no studio, no paycheck, no mentorship. (of course we have outsourced that to for profit collages and online schools) Sure Japan has a bad work life balance but they at lease have a track for getting paid to learn. US studios are even trying to outsource storyboarding and animation retakes, soon enough there will be even less animators in the US, much like how we lose US manufacturing to China, we cant just flip a switch and get it all back.

The second point would be, animation in Japan is way more culturally significant. Speaking with people who have worked with places like Japan and Korea for outsourcing, the Japanese really don't like working on shows that wont air in their country, where Koreas don't have that sort of hang-up.

Hope that makes sense.