They are made from a material that is easy to reuse - steel.
In fact you can't make modern furnace steel without adding scrap steel to the blast furnace to control the temperature.<p>The problem is that the floors of the containers are wood and to protect the farmers they are impregnated with some of the nastiest pesticides you can imagine - so no foreign bug can hitch a lift.<p>Used containers are basically toxic waste, if you are reusing them for housing/storage/etc you are supposed to remove the floor, sandblast the inside and repaint everything in an EPA approved paint - which costs several times as much as the container.