Decades of government backed protectionism paired with an ageing population will do that. people were widely propagandised into xenophobia and that sort of thing tends to stick in the psyche of the community for generations.
That said, the older generations are the ones still holding onto those views and they will be forced to change eventually.
You can still find signs on restaurants or shops across Asia that say “no tourists” or things like that, but they are becoming less common, even in rural areas. there is still the language barrier with the older generations, which is part of the reason those signs existed, but the majority of younger people across most of east Asia have some level of English from their mandatory school curriculums. They learn more western history, more western customs, exposed to more western media, western style homes are popular to those who can afford them (the Japanese housing market is it’s own deep, deeeep topic), etc. etc. so those people naturally become more open and accepting of immigration
I expect to see japan keep crashing for another 10 years or so though sadly, and while Korea has been fairly stable they are rolling towards the same sort of downturn themselves.
China has been slowing economically for some years now for the same reasons, but their situation is a little different as their government will do whatever is necessary to protect their image, above their actual economy, so it is hard to know what is really happening there. For example, the whole giving gold to home buyers to avoid crashing the housing market thing.