Why are aircraft external lights round, not tear-drop shaped?
That beacon lens (and most of the strobe lenses shaped like it) are actually Fresnel lenses.
They are optically designed to produce a specific plane &amp; intensity of coverage, and tinted for a specific color. (The color, intensity, and plane are specified in the FARs &amp; applicable Technical Standard Orders if you want to go hunt down the requirements.)  

The lens optics are easier to see on the split (Red/White) lenses:


A Fresnel lens is not the only way to meet these requirements, some anti-collision lights do it by sheer intensity (while retaining the "jar" shape, mainly for installation compatibility), and as GdD pointed out there are more aerodynamic options available. The Fresnel lens method is usually adequate for smaller/slower aircraft however, and the drag penalty at relatively low speeds isn't significant enough for designers to worry about.