Why are JDM engines available at all?

[finaltable]

Pardon my ignorance, but why are there so many engines from Japan available for purchase? The idea of getting a 1JZ-GTE engine + 6 sp transmission for so little seems like a great bargain, even if it costs me $2K. I am assuming that there is some reason these can't be used in Japan (why export engines we can't even get in a car here if they could be used?) and since they seem to have only 30-40K miles it looks like they get about 2 or 3 good years of use (although 10-15K miles a year on an island seems a long way) and then they are sold. I truly doubt the reverse market exists because I have yet to put less than 100K miles on a car. Why is it just the engines? Are they putting new engines into the existing cars from which these are being pulled? All the websites for JDM engines I have looked at appear to take it for granted that the visitor knows why this market exists because they just go straight to inventory/pricing.

An explanation of the situation in Japan from which we benefit would be appreciated.