Audio-in Jacks in Cars

Why don't all carmakers include audio-in jacks in their cars? These days we all have so many handheld music devices such as portable CD and MP3 players, while no car comes without a sound system. Why not be able to plug the former into the latter directly, without having to use yet another piece of kit, i.e., ungainly intermediaries like cassette connectors and short-distance radio transmitters, to hook up the two? And while they are at it, car makers could replace/supplement the cigarette lighter socket with one that does require yet another adaptor for charging phones, players, laptops, &c.