I'd guess it's because although cars do operate at this altitude they rarely descend at such a rate on a low power setting.
Assuming you could descend at 500fpm in a car, you would be caning it to get the descent speed or if you were driving normally then you would be in a lower gear with higher revs - both of which are in a higher power setting which is the opposite to what is required to get carb icing - a low power setting.
Plus, I reckon the engine bay in a car would be warmer, in particular the induction area, than an aircraft's.
Now, I'll just sit back and wait for Evo, Beagle or 2Donkeys to come along and blow my theory out of the water.
