Earthing is probably just as necessary for cars as it is for aircraft, but isn't done because, as has been pointed out, virtually all drivers wouldn't bother, and there should be some measure of earthing through the tyres. I am not sure about aircraft tyres having as much carbon content. About three years ago there was a massive recall on Vauxhall (GM) Astras to bond the filler neck to the car body, as the lack of a bond caused an explosion/fire at a filling station in Germany with, I think, fatal results. More important for aircraft because the consequences are far more severe: more fuel, more people, aircraft more expensive than cars etc.