Guys remember the good old days when men could throw on a tunic, or strap on what looked like your best dressy white bed sheets and a bit of rope and you were good to go fashion wise for the entire year? What happened to the good old Roman days where men could essentially wear togas and tunics? Why can't men still don these most comfortable garments without fear of ridicule and why is it that women are allowed to wear "masculine pants" and men not dabble in women's wear?
Well, for men, horses, war, and your need for power happened and drove your legs straight into pairs of pants for life. Ages ago, in a world without A/C and sewing machines, it would have been really impractical, hot, and time consuming to put on or create a pair of pants. Then along came a lot of war, like a lot of it world wide, and a strange phenomenon happened. Those groups of fighters who donned pants in infantries for practical utilitarian reasons tended to win far more often and dramatically then those who donned skirts, kilts, togas, or some version of. And who doesn't like to win at war?
Pants were seen as the key to not only status, power, and winning, but as a practical garment for long uncomfortable or weather weary rides..not to mention pockets for storage. Seeing as how it would be a long time before cars came into being, pants just became the practical masculine powerful thing for men to wear and it stuck...clearly because you guys have been wearing them ever since.
Contrast this with the ladies of the time. They were either simply not allowed to ride and certainly, not to fight. When they were allowed to ride horses, they were made to do so side saddle. Ironically horses, and war, once again would soon become some of the reasons why women eventually came to wear more masculine clothing and pants in particular. Side saddle is f'in dangerous. You're a lady on a horse in 15 pounds of skirts, hoops, corsets, and crinolines. A horse gets spooked, bucks you off, you get trampled, mangled, injured, or you're killed. Secondly all of the weight and baggage of the old fashionable skirts and dresses would sweep up street trash, became a fire hazard, and led to injury with tripping down stairs especially in inclement weather and were impractical to do much in but sit and stand. When the first bikes came into existence, a pair of bloomers came with it to assist women in their travels, but once off the bike, it was back to her skirts.
Pants officially started making their most shocking mark in women's fashion in 1911, courtesy of French designer Paul Poiret, who created the harem pant. Then in the 30s a few major leading ladies in pictures wore them influencing the trend, but they were still only worn by a few, some rabble rousers, and never formally.
Once again war breaks out and women this time are at home and abroad aiding in the war effort during WWII. Female pilots wore pants and it became ridiculous and impractical for women to work in tight spaces all day long with heavy dangerous machinery without pockets or the ability to move about unencumbered and such wearing a skirt cut to the knee. But once the war ended, there was no more need for factory war effort, and with the most feminine Dior look curling it's fashionable influence around women once more, dresses and skirts were back in again.
Trot on down to the 1960s-70s when the sexual revolution and feminism part 2 happened, the 80s where women began to really in earnest start entering into the male dominated board rooms of America and took on the masculine profile of power driven linebackers with big shoulder pads, and to the 90s where jeans were feminized and sexualized and made cool by popular teen television and you have your answer as to why they are a staple for women today.
We know why women aren't going back to a world filled with dress and skirt dress codes only, but what about men? Why have they not gone back to free flowing toga styles or why don't they throw on the occasional dress? Well, unlike for women wearing pants now, there is no practical reason for it. Wearing a dress/skirt type clothing style would not help men improve upon their jobs, sports, war, or outdoor hobbies. They would in fact probably cause the same exact safety concerns that women had with them which is why women tend not to wear them when doing the same activities. Secondly, a/c has been invented, so unlike the Roman days where it would have been more practical to wear a loose free toga or tunic in the baking sun, the same issue does not exist today. Finally men simply don't want to. Men seemingly place a much higher value on being seen by both men and women as being strictly masculine as defined by cultural norms. By today's standards, the idea of wearing pants is a masculine one even though few men associate women as being masculine who wear pants or shorts unless those women never wear anything but pants.