Japan and South Korea have become stagnant since the 2000s. The teams that are showing the maximum improvement from Asia are Saudi Arabia, Australia and Iraq. And some of the most promising ones are United Arab Emirates and Uzbekistan. In terms of ranking, Japan, South Korea and Iran are at the top, but I am not witnessing much improvement in the quality of their game for the past many years. Both Japan and South Korea win a match each against stronger sides in Qatar 2022, but they found it hard to maintain momentum.
The reason why Japan and South Korea could get more progress is the plan they got and the hard work they do to stick to the plan they have. It is still quite common for other teams and countries like Saudi Arabia to spend much more money than other teams in similar fields in Asia. This is why, based on the investment they have made in these years, it is quite normal for them to be showing more improvement as compared to other teams.
I think it's because Japan and South Korea is not giving any priorities nor funds to their football clubs because the said sports didn't belong to the country's top favorites, I mean, we can see it in the SEA games where their soccer/football team is really not that good. Aside from that, the named countries are taking more focus on their economic growth as they don't have that much funds like the Saudi Arabia does where they have the luxury to spend millions on their national team.