"After the demise of the multi-ethnic Empire of Japan in 1945, successive governments had forged a single Japanese identity by advocating monoculturalism and denying the existence of more than one ethnic group in Japan. [...] the notion of ethnic homogeneity was so ingrained in Japan, to which the former Prime Minister Taro Aso (1940-), in 2020, notably claimed in an election campaign speech that “No other country but this one has lasted for as long as 2,000 years with one language, one ethnic group and one dynasty”." [1]

They have a long history of "Aryan-like" master race theory [2].

Why would the Japanese want to trade in their distinctiveness and positive societal attributes for dubious gain? It endangers the fabric of society.

Where has mass migration been a net benefit for a larger country in recent times? Europe has been "culturally enriched", which puts even more pressure on the already strained welfare-, medical-, education- and housing-system, lessening the quality for all. There has been an increase in savage violence, such as gang rapes and brutal attacks, as well as organized crime, terrorism and epic fraud the institutions are not equipped to handle. Outrageous sums of tax payer money is used to pay off foreign countries, so that they throttle the influx of migrants somewhat, which also makes Europe vulnerable to blackmail by those countries. The people do not want (incompatible or malicious) foreigners. The governments are incompetent in letting in only the good ones and getting rid of the bad ones.

[1] https://en.wikipedia.org/wiki/Ethnic_groups_of_Japan#Notion_...

[2] https://en.wikipedia.org/wiki/Racism_in_Japan#History