Since ancient times, trousers have been worn in the western world. Trousers were first invented because robes are uncomfortable to wear on horseback. In fact, those who wore trousers in battle had the upper hand over those who wore robes. For this reason, civilisations all over the world adopted this form of dress to survive battle.
They are now the most common form of lower body clothing. Practical and comfortable, it is no wonder that they have been adopted by both sexes as part of their daily clothing.