Steve points out what many scholars have said over the years: most of the world originally wore unbifurcated clothing of some sort. Granted, many cultures had simple long tunics (which are still worn in the Middle East, for example). In the Scottish Highlands, it would seem to make sense that they wore a base tunic or long shirt and added a thick layer of warm layered wool over that, out of necessity. Some of the earliest descriptions of Highlanders mention that they wore their plaids as folded mantles, then unfolded and wrapped themselves completely in their plaids to sleep, etc., but would also cast them aside to fight or to engage in other physical activity where the plaid would be cumbersome. The great kilt was likely just the simplest way to provide great warmth when needed, but also give them the versatility to shed it quickly. Rather than tailoring it into a close-fitting shaped garment, they chose to keep it as basically a big blanket that could be reconfigured to a multitude of uses. For their purposes, it was a do-everything item. That's very sensible for the amount of traveling by foot that they did.