Like any other country, men in Saudi Arabia wear jeans and aT-shirt, but they have their traditional clothing called a Thobe. The Thobe is made out of cotton and it is long sleves and it's down to the ankle. It bottons down the middle and also has a pair of white cotton pants that are worn underneath. Thobes are a more formal garmet and are worn in goverment offices, schools, and big events like weddings. Its designed to keep you cool in the heat of Saudi Arabia. Men also wear a Shumagg, which is a red and white checkered scarf worn over their heads. This is used to protect the head from direct sunlight and can also be used to cover the face and mouth during a sand storm.
It says in the Quran that men should be judged by their deeds and not by their appearance and that men should feel equal. This is why all men's traditional wear looks the same. Another reason is simple because men in Saudi Arabia have worn thobes for hundreds of years, its tradition.