If you mostly get around on your feet, distance traveled in a day is a reasonable metric for how much exercise you got. It's true that it also matters how you walk and where you walk, but it would be pretty tedious to tell someone that a "3 mile run" is meaningless and they must track cardiovascular health directly. It's fine, it works OK for most purposes, not every metric has to be perfect.
But once you buy a car, the metric completely decouples, and no longer points towards your original fitness goals even a tiny bit. It's not that cars are useless, or that driving has a magic slowdown factor that just so happens to compensate for your increased distance travelled. The distance just doesn't have anything to do with the exercise except by a contingent link that's been broken.
