In general the most aero-dynamic (or hydro-dynamic shape) is a rounded front and a tapered rear (this is not true when close to speed of sound).
If you look carefully a lot of objects are like this, right down to fish which have round fronts and taper off to a thin tail. Also not only are wings like this, so are keels and rudders on yachts etc.
Race cars are not because downforce is more important, but low speed high efficiency vehicles (pedal driven endurance tests, minimum energy vehicles etc) are the same shape