Yeah, what everyone else has said - to increase the apparent value of the car, to sell expensive stereo options etc. They are also still learning about car systems integration too - satnav, roadside assistance, security, general controls etc.

But this is a short-term issue IMO. At some time in the near future (August??) car manufacturers have to open up more of their protocols to the general public, so OEM manufacturers will be able to start learning about integration themselves. In addition, once the car manufacturers realise that they may be losing potential sales due to their particular brand of stereo not being up to snuff then they will probably start to have second thoughts about making it so hard to replace. For example, assuming that Ford and VW had 2 otherwise equally desirable cars (desirable for your needs), are you likely to buy the VW with it's Phatbox mp3 system, or the Ford with it's Visteon (empeg) system?