> The most satisfying answer in the end? Because the car designer decided not to put one there. Exactly why they made that decision could well have to do with any of the speculations above, as determined by their unique set of traditions, values and sensibilities.
Unless it's a hatchback or SUV, there isn't really much of a problem with the rear window getting dirty. Rear wipers end up looking awkward and weird, and can't even make up for it by being useful.
Not all read windows are placed at angles that need them. To ask this question implies you are letting the giant toaster model of vehicle obscure your memory that once there were vehicles shaped like beans and before them were actually appealing sleek muscle cars that guzzled the dinosaur juice like no tomorrow but looked like what we all spaceships to look like when the reality is shaping up to be even larger toasters that expand to full size only when in orbit.