For that matter, why does a mirror reflect images from side to side and not top to bottom? And how come greater tension in a guitar string gives off a higher pitch instead of lower? Three landmark questions, all of which have clearly been at the top of your mind for at least the last 61 years—and none with which anybody can help you, poor thing. The answers may illustrate certain laws of physics and physiognomy, but they don't come anywhere near explaining them.
It's sort of like trying to justify the punt, that crazy ol' dimple at the bottom of a wine vessel. Pick up your bottle of Riesling for a second—chances are it has a fairly deep concavity at its base, the existence of which is the stuff of some fairly cool theories. Some say it provides greater structural integrity to the bottle and prevents it from tipping; others suggest it's a vestige of ancient glass-blowing techniques that's hung around for posterity's sake. Maybe the neatest take of all involves some subtlety of communication. In the old days, household servants were said to know more than their masters about a guest's integrity; the servant silently vouched for the visitor's honesty by sticking his or her thumb into the punt.
Pretty fun lore, all right, but today's glassmakers say that's all it is. Technology, they insist, has rendered the punt unnecessary and that its persistence is a mystery. You might want to note that the next time you stock up on your wine of choice. Good, inexpensive wine is all over the place, no matter its color or concentration—and a flatter bottom to the bottle may indicate an even greater bang for the buck inside.