We all Japan is the power house and home to famous vehicle manufactures like Honda, Toyota, Nissan, Mazda, Mitsuishi. These giant automakers have set an example and set standards for other automakers in the US & China to follow. Outstanding quality & performance of Japanese motors result in fuel efficient vehicles.
Hybrid technology used in Toyota Prius engines, Nissan Leaf are great examples. Japanese manufacturers are great innovators. They set the best example. Quality is the key. Greener technology paves the way for future. All this makes Japanese automakers the leaders in automotive industry.
Now lets talk about used engines & used autoparts. Japanese government is very proud of automobile manufacturers at home. In order to get an edge and control exports they have strict transportation rules & regulations that force Japanese people to stop driving within 5 years. Japanese culture has been like this for over 10 decades. They don't have to force the public, they willingly do it. Propensity of a Japanese to drive new cars is also another conducive reason why they give up on their vehicles withing 5 years.
Japanese driving habits are the main reasons that result in availability of Low mileage Japanese Engines
In Japan they rarely drive on weekdays due to expensive parking, fuel costs ( which is 20% more per litre than in the USA) & cheaper alternative; which is public transport. It is cheaper to travel by shinkansen ( bullet trains), subway and other means of transport. For example a ticket form Osaka to Tokyo will cost just $100 which is far cheaper than driving your own car. This is the reason why most Japanese drive short distances. They mostly drive on weekends.
I would commend the Japanese automotive industry and Japanese Government for supporting their manufacturers. This is the reason why there is great demand for used parts & low mileage Japanese motors in the world.