This is solely due to Japan´s obsession in wanting to remain a country with one of the lowest percentages of foreigners in the world. However, given that climate change will displace hundreds of millions of people, many of them in Asia, it would be advised that Japan would change its attitude and be prepared to receive a few million. Even because Japan, too. has contributed very actively in terms of carbon emissions to cause the rise of sea level which causes climate refugees. The publication in the link below has calculated the responsibilities of the 20 most "polluting" countries showing that Japan is morally obliged to receive almost 6% of climate refugees.
Not to mention the obvious, that in Japan they seem to have forgotten that with age all the human body withers away and not just in muscle terms!
