My ...is that one reason is because Japanese are able to get an earlier start.
In other words, most Japanese live at home for a longer period of time than e.g.
Americans, often until they are married or even later. This is just a guess from my own experience, but after I left home for college I was basically on my own, and when I moved out of my home town to start a career I was a pauper for years with rent being my number one expense. Many Japanese don't pay rent since they live at home, and therefore can start saving sooner. Although interest rates here in Japan are ridiculously low compared to other countries, the compounding effect kicks in, so the earlier you start, the better off you'll be after a number of years.

Bill