Cause the point of a car is to get you places, not to screw around with the internet.
And the point of a cellphone is to make and receive calls. That didn't stop us from using them for dozens of other things.
And why wouldn't you want to screw around on the Internet? You don't want news on demand? Traffic and weather reports? Maybe Google Latitude or some "Find My Car" app so if you can't remember where you parked (or your car gets stolen) you can track it from your smartphone?
I wouldn't mind being able to visit websites like tomorrowsgaspricetoday.com in a hands-free manner without having to break the law by operating my smartphone on the road.
Not to mention Skype video-calling. That could easily be done.
I wouldn't mind syncing of my music collection (or cloud music service) from home, to phone, to car, etc.
Plus maybe I don't want to listen to the radio on the way to work, maybe I want to listen to a YouTube vlog.
The hardware for all of this stuff is cheap, as evidenced by the $500 smartphones. You could easily share minutes and share your data plan with your cellphone.
People already pay for things like this in their cars, but they overpay thousands of dollars to get something far inferior.