Because most people want the sound "imaging" in front of you. When you listen to your stereo at home, you are useually looking at it, not turned around so that you hear it from the rear. In my Impala, I installed some expensive tweeters in the outboard airvents, mounted to the "floor" of the airducts. Totaly invisable, yet the difference in sound stage and clarity is very evident when you compare to my 9C1 that does not have this feature (but otherwise similar speakers). I highly recommend placing the tweets there! They were kind of a pita to put there though. BTW, these tweets were in addition to my component setup in the doors, mounted `ala HACKS template. I installed the tweets (kicker nd22's) in the vents just because I had them, and if it did not sound good, I would just dissconect them and be done with 'em. They had a pretty elaborate crossover with them that also went up and under the dash. I also played quite a bit with the phase, trying to them both ways, and IIRC, finially settled for them 180 out of phase.
The upshot is, the sound stage is elevated by a noticable amount, and everyone who hears it, likes it.