Why are there only the East Palace and the West Palace, but not the Nangong Gong Bei?

Three palaces and six courtyards is a Chinese character, which comes from the architecture of the Forbidden City. The "three palaces and six courtyards" are all in the palace of the Forbidden City. Generally refers to the imperial concubine. The "Three Palaces", also known as the "Last Three Palaces", refers to the Gan Qing Palace where the emperor lived, the Kunning Palace where the queen lived, and the Jiaotai Hall between Gan Qing Palace and Kunning Palace (the name of the hall is taken from the Book of Changes, which means "harmony between heaven and earth, health and happiness"). The iron sign "Inner Palace does not interfere in government affairs" stands on this hall. ) The Sixth Hospital is actually the Twelfth Hospital. There are six palaces in the "Three Palaces", namely Ren Jing Palace, Chenggan Palace, Zhongcui Palace, Jingyang Palace, Yonghe Palace and Yan Xi Palace. The "Six Palaces on West Road" are Yongshou Palace, Yikun Palace, Palace of Gathered Elegance, Xianfu Palace, Changchun Palace and Taiji Palace (Qixiang Palace). Because all palaces are quadrangular buildings, they are called "East Sixth Courtyard" and "West Sixth Courtyard". Walking north and south, you can't build a palace, which is contrary to architecture and geomantic omen.