Part of the reason the USA is cutthroat and poorly run is because of how heterogeneous the population is and the frontier mindset that comes with that. Importing immigrants en masse is likely to cause a lot of social instability and isn't a decision that should be taken lightly.

Think about it, all the hallmarks of Japanese society, tradition, respect, how will those survive if there's a relentless onslaught of foreigners who do not understand or believe in those values. Part of what made Japan was their ability to guard themselves from foreign encroachment and only incorporate elements of western culture that worked for them.