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1. Optimum aero is not the only (or primary) design criterion in transportation, even air transport. The fuselage of a commercial airliner looks like a rounded barrell because that is one of the easiest shapes to pressurize without fatigue stresses downing the airliner after a few thousand "cycles."
(Also notice that "new" designs often have extended fuselages based on previous designs, due to the fact that the previous design has already gone through certification, and a modified design can be put into production more cheaply and quickly. Example: DC-9 and MD-80.)
2. Pointy noses are best when supersonic, as the air ahead of the vehicle "doesn't know what's coming" and slams into the leading edge. Subsonic, the air starts getting out of the way before the object ever gets there.
(A pointy leading edge isn't all that bad, subsonic, provided you never, ever, have a crosswind. Pointed leading edges are very intolerant of wind hitting it sideways.)
3. A teardrop, of defined "fineness ratio," is the optimum shape to enclose a given volume. Usually, that isn't the primary concern: the P-51 was designed to have a frontal area just large enough to encapsulate the pilot. This made for a plane that was "too skinny," in terms of fineness area, for optimal aero, but it wouldn't make sense to make the plane "fatter" just to meet the teardrop ideal--humans aren't fluids, and you can't "make up" for a deficiency in one dimension by making a surplus in another!
(Now, a auxillary fuel tank encloses a fluid, and thus makes an excellent candidate for a teardrop shape.)
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