Is the answer because they are healthy people?
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https://en.wikipedia.org/wiki/Health_in_Japan
The level of health in Japan is due to a number of factors including cultural habits, isolation, and a universal health care system. John Creighton Campbell, a professor at the University of Michigan and Tokyo University, told the New York Times in 2009 that Japanese people are the healthiest group on the planet. Japanese visit a doctor nearly 14 times a year, more than four times as often as Americans. Life expectancy in 2013 was 83.3 years - among the highest on the planet.
Cripes, I don't think I've been that many times in my life.
Mark