This is not a fair comparison. When you drive a car, your eyes are your primary "instrument", you're travelling much slower than a plane, your margin of error is much higher, you can always slow down or stop if something is wrong, and making a single mistake doesn't mean a high risk that you and all your passengers will be seriously hurt or killed.
When you're in a plane you rely on instruments to tell you where you are, what direction you're pointing, and how to line up for your landing. On a good day your eyes can tell you much of the same information, but on a bad day or at night you rely very much on the instruments. As any small child learns in school, instruments such as compasses can be affected by other (electro)magnetic fields nearby. Who knows what other instruments on board could be affected by RF emissions as well.
I think the problem is not that most iPhones or electronic devices are "unsafe". My iPhone has a built-in compass after all, and it wouldn't work if the phone itself was emitting that much noise.
I suspect that 99.999% of consumer devices wouldn't pose a problem. The problem is that if you make a ruling that allows all electronics, how do you know that one day, somewhere, someone isn't going to bring aboard that 0.001% device that is leaking RF or EM, either because it was badly made or defective? And maybe that happens to be the flight on a plane that's older and has damaged or insufficient shielding, and it's a night-time, stormy, zero-visibility landing...
Slim chance, yes, but how slim is enough? The FAA aren't exactly known for taking risks.
