Even with its aging population, the population of Japan remains 98% Japanese. Why is this?

In Britain, we are solemnly assured that to cope with our increasingly elderly population, it is vital to invite millions of immigrants to come and live in our country forever. Japan manages to avoid this situation. #:~:text=Japanese 97.9%, Chinese 0.6%,Brazilian) (2017 est.)&text=The World Factbook, the indispensable source for basic information.&text=Shintoism 69%, Buddhism 66.7%,6.2% (2018 est.)