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.

[tone loc]

im sure its just engines because of the left hand drive thing and other reasons to. In japan its almost unreal to own a car. you have to pay all kinds of taxes emmisions is alot more exspensive so thats why they only drive the car for a couple years because the older the car the more money you got to pay

[Supra2NR]

after 30k, thier insurance doubles and all other hassle of owning an older car
(yes, 30k is old for them)
so the smartest choice will be to junk the car since nobody else wants to but them

thats where all the front clip, jdm engines come from
basically we're recycling for them

[finaltable]

Wow. I can't remember the last time I had a car with FEWER than 30K miles on it. Thanks for the info.