What is the fundamental difference between the flute geometry of an end mill as opposed to a drill bit? Obviously drill bits are not intended to cut horizontally so what difference about them makes them unable to do this efficiently? Also, since end mills can cut vertically, why not just manufacture drill bits with the same flute geometry as end mills?
Take this all as very general....Flutes of a drill bit are ground only for chip extraction, where as end mills take the chip extraction flute and grind an actual cutting edge on it ( b - c ) and some relief (c - d). Picture is of a 4 flute end mill, the one on the left being proper. There's MUCH much more to cutting tool geometry than this brief explanation, a cutting tool maker or a book on tool & cutter grinders will do a much better job explaining it.
And as far as grinding cutting edges on the flutes of a drill, for one it would make drills much more expensive to manufacture. In manufacturing environments, end mills are generally re-sharpened, for me, any drill under 1/2" gets replaced. Drills are also generally held in drill chucks, end mills in collets or mill holders. Drill chucks are not designed to withstand the radial cutting forces of side milling so a cutting edge on a drill flute would give a false sense of acceptable practice. And one more...the ratio of diameter of the tool to the length of cut is also much less with an endmill, IE, a 1/2" endmill LOC is usually around 1" where a 3/16" drill can cut 2" or so deep. That added LOC of a drill substantially weakens it in the radial direction. So standard, jobbers length drills would deflect far too much to be a usable side miller. Just some examples for you.
Some drills have somewhat of a cutting edge ground into the flute, but from my experience, its for helping cut materials that tend to "close up" on you as you drill deep into a hole, bronze/copper/etc.