The distance that snow can fall off and hit your vehicle can be longer than a safe gap, and in any case, you should generally not be driving in such a way as to rely on other people driving perfectly safely except where it’s impractical not to. Since it’s perfectly practical to fully clear vehicles of snow, that’s what ya gotta do.

You can see it the same way as pulling out in front of someone. Sure, unless you’re a complete moron, doing so rarely results in a crash, because the other driver will brake to avoid hitting you. But every once in a while that driver will be distracted, or unable to react in time, or be unable to slow down as well as you thought. Or, you can see it as slowing down when passing parked cars in narrow situations: everyone should be behaving sensibly and so you should be able to maintain a high speed, but all it takes is someone to step out from behind a car without looking or an oncoming vehicle to deviate from their lane and you have a disaster.