There are two most prominent explanations for why the rear car window doesn't go down all the way. The first is in the structure of the car and the door... if you look at the rear door of many cars, the rear wheel well extends into the bottom of the door, and in fact there is not enough room for the window to roll all the way down.
This explanation actually makes some sense. But still, it bothers me. The engineers who design cars are pretty smart people (I imagine). They know how to make a two ton piece of metal that can travel upwards of 100 mph, under the control of a couple foot pedals and a steering wheel... very impressive engineering, there is no way I could do that.
You're telling me that these incredibly smart engineers can't figure out how get around the shape of the freaking wheel well?!? See, this is why car companies like GM are failing. If GM made cars with rear windows that rolled down all the way, they would never have been forced to declare bankruptcy.
The second reason the rear windows do not roll down all the way is as a safety and liability issue. Apparently, the car companies are afraid that little kids are going to climb out of the back window of a moving car. This explanation is laughable, and raises numerous issues.
First of all, aren't little kids supposed to be in child safety seats? Every state in the country has a law saying kids have to be in safety seats when they are very young (the exact age or size at which this requirement ends varies by state, but they all have something). If the kid is in a child safety seat, they probably can not escape and start trying to climb out the window.
Secondly, if the kid really, really, wants to climb out the window of the moving car, and neglectful parents forgot about the child safety seat, is the little bit of window that is sticking out actually going to stop them from escaping? It is not an electric fence, it is not topped off with barbed wire... it is a thin piece of safety glass, it is not hard to grab ahold of. If you really are worried about kids climbing out the window, you should be more careful developing car seats that are unescapable.
Thirdly, and most importantly in my mind, what kind of kid tries to climb out of the window of a moving car?!?! I mean, kids do some dumb things, but climbing out of a moving car? I don't think so. If you do have kids that are trying to climb out of the rear window, that on its own is probably the least of your problems. You should be focusing on the real issues, like, "Is my kid insane?" Or, "Do they make baby doses of Valium?" (The thought of little kids on drugs makes me laugh... does that make me a bad person?) Maybe most importantly, "Do they make straitjackets in a size XS?" Hell, maybe you should call up your friendly neighborhood exorcist to solve the baby's problem... "The power of Christ compels you... to stop being a dumbass!"