Why didn't China bring Mobei into the territory?

Because the Han nationality is a farming nation.

If you want to rule Mobei, then long-term garrison and immigration are essential. However, the climate of the Mongolian plateau is unbearable for residents from the Central Plains. The local climate type belongs to temperate continental climate, located on the plateau, and the atmosphere is thin. In summer, ultraviolet rays are strong, the sun is exposed, the temperature difference between day and night is large, and the temperature in winter is extremely low. There are often large-scale snowstorms, which is a great challenge to human survival. Moreover, the Mongolian Plateau is dominated by the Gobi Desert, and the land is relatively barren. It is difficult for Han Chinese who moved to their native land to carry out agricultural reclamation, so it is difficult to carry out military reclamation or civilian settlement in the local area. Therefore, long-term garrison management is an idiotic dream.