Still, I expect the numbers from Asian countries to steadily increase—unless we go into some pandemic lockdown again.
Chinese university entrance exams can be hard or easy, but in either case it doesn't really matter because admissions are done on a quota system. When I was looking at score data a few years ago, the threshold for being admitted to a second-tier (二本) university was around the 40th percentile, which is to say that if 55% of people in your province are better than you, you're still comfortably going to get into a university.
Individual schools may of course apply much higher thresholds (for example, there's also a first tier!), but at that point you really have to admit that the difficulty of the standardized test is irrelevant. What matters is how you score relative to everyone else. The point of the test is to have a high enough ceiling ("be difficult enough") that you can tell the difference between someone at the 99.9th percentile and someone at the much higher 99.98th. The optimal test for that purpose is infinitely difficult, but you need to balance against time requirements, testees getting demoralized and giving up, etc.
Gaokao et. al. are held once a year, if you want to retry you need to wait a full calendar year, and the single-time test covers all subjects simultaneously. To compare to SATs, those can be retaken multiple times a year, and the non-core subjects are separate, so any individual sitting is much lower stakes.
It is not exactly a secret that a big push factor for Chinese and South Korean university students is how competitive their local systems are.
You don't get to know your gaokao score when you apply to schools.
But you do know what your score is likely to be; that's how people choose what school to apply to. It's not very difficult to predict your score in advance of taking the test.
Given that, why do we think the annual frequency makes the test "high-stakes"? The normal case is that you go in expecting a certain score and get something close to it. If you wanted to take it again the same year, you'd expect a similar score. If you wanted to spread it out over several days... you'd expect a similar score. There just isn't a large population of people who perform one way on practice gaokaos that are very similar to the real gaokao, and markedly differently on real gaokaos that are very similar to practice ones.
It's no coincidence that the biggest groups of immigrants to Japan are the Chinese and Vietnamese, who have a leg up on the language (Chinese characters for the Chinese, obviously, but Vietnamese has a lot of Chinese loans shared with Japanese too) and blend in better in terms of physical appearance and culture.
There are like 50K Indians in Japan, mostly Tokyo. This is a surprisingly low number. Sure there is the language barrier but that barrier holds true for Germany also which has a lot more Indians.
My hunch is education - Japan does not have too many universities to attract young talent, as compared to Germany, Australia or the UK. I'm not sure if this is by design or if there are some other factors at play.
If Japan built a lot more universities and made it easier and relatively affordable for foreign students to sign up with English classes etc, perhaps there will be a lot more immigrants from countries such as India, Vietnam, Cambodia etc.