The reason you find lower mileage JDM engines has more to do with emissions testing. Once the vehicle is ten years old it requires yearly inspection which is very costly so most Japanese people just get rid of them. The problem with knowing you're going to get rid of it at the 10 year mark can lead to some engines that are truly junk from all the sludge buildup due to lack of maintenance. I'm not saying it's the end of the world, but it is something to consider when buying a JDM engine. Also, the JDM 5VZ engines usually come from Hiace vans and require swapping over a lot of components from your original engine to the JDM engine.

By the way Japan is almost as large as the entire East Coast of the US.