Greatest pants ever, Karnataka, India.
Why does western civilization wear pants? Why not kilts, sarongs or robes? Why not togas? Two insightful pieces (part one, two) say that it was horses. Not that horses wore pants, but that men riding horses did.
In modern times you may encounter this if you try to ride a motorbike in a sarong. It’s not easy, or especially attractive, unless you loop the back-bottom of the sarong up, making something akin to pants. Or diapers.
The argument, then, is that the switch from the tunic to trousers in the civilized Europe followed the replacement of the hoplite by the knight. Historically there is a very strong correlation between horse-riding and pants. In Japan, for example, the traditional dress is kimono, but the warrior class (samurai) wore baggy pants (sometimes characterized as a divided skirt), hakama. Before the introduction of horses by Europeans (actually, re-introduction – horses were native to North America, but were hunted to extinction when humans first arrived there), civilized Amerindians wore kilts: But when the Plains Indians started riding horses they also adopted pants. (Social Evolution Forum, Part 2)
Seriously, read part two at least of these posts by Peter Turchin. Fascinating stuff. Part one covers why sarongs and kilts and tunics are better (for warm climates at least):
Male’s testes hang outside the body for a reason: the optimum temperature for spermatogenesis is a couple of degrees less than the body temperature. So wearing tight pants kinda defeats that purpose.(Social Evolution Forum, Part 1)
Yet pants took over, because men in pants (atop horses) were much better at killing men in comfortable robes. I guess the ability to throw a spear down has a higher evolutionary advantage than producing a bit more sperm. So social convention followed.