Why men wear trouser and women wear skirt

In the beginning, everyone wore skirts. In Egypt, Pharaoh, his subjects to the simple citizens wore skirts of fine linen. In China, the emperor would wear a bright yellow, silk, floor-length dress. In South America, they all wore skirts. In ancient Greece and Rome, men and women wore tunics. Tunik is a dress draped around the body with a cape and all there is held at the shoulder. Men's dresses were shorter and females were longer at the ankle area. Everyone was wearing a skirt and gown, and this was normal. If you aren't wearing a skirt, you were odd. So where did this classification start?

Two factors, theorists note, determined the use of pants by either gender: cold and the necessity for horse riding. Pants were created for these two reasons in those days, not only for the male gender.

The influence of the Bible (Deut 22:5):

Only men should go to war and men shouldn't escape going to war by putting on women's clothing or garment, lest they are mistaken for a woman and not allowed to go to war.

King James 1 of England (1604) ordered the Bible to be translated into English, which is easier to read than the original version. It was further translated into the King James version and the New International Version, and by then, the translation was almost varying from what the original text meant.

Since the new translation didn't put it view that the statement was directed to a war scene, so many statements and derivations were made and from that point in time, men's clothing and women's clothing started becoming distinct and different.

This was separate clothing for the two genders. It was honestly at an advantage because what each sex wore suited whatever they did. Trousers looked perfect for the activities men partook in, which include going to work, going to war, riding horses, etc.
Horse riding in many of these cultures originated as wartime activities primarily practiced by men.

In those days, women weren't required to do much and their work seem suitable to do in a skirt. Being pregnant, using the toilet, and all that. Even during their menstrual flow period, they would wear rags between their legs to catch the blood and all these were just suitable and comfortable to do in skirts.
Women (who were more into living inside homes) could wear skirts, but men had to wear socks to stop the cold. With trousers, you don't have to have long socks–and also they are good for moving around.

Hence, men wear trousers and women wear dresses.

It became a culture over the century until everybody saw it as a normal thing.

The entire idea of skirts as the specifically female dress was brought about in European thought by a combination of tailoring innovation and particular occupations; and the tradition of men in cloth wraps, skirts, tunics, robes, and other non-pant items is vast and ancient.