Why didn't Hetao area belong to Cao Wei during the Three Kingdoms period?

This should be the map in the middle school history book! It's just a schematic diagram, not very accurate. In fact, in the Eastern Han Dynasty, the Qianghu area near the eastern part of the map should have belonged to Shuozhou at that time, while the western part belonged to the area where Qiang people lived (note that it was Qiang people, not Xiongnu). At that time, the Han Dynasty set up a Qiang protection department and a captain to rule the Qiang people here, and the location should be in Jincheng County. At the end of the Eastern Han Dynasty, Bian Zhang and Han Sui rebelled in Jincheng, but later they accepted the favor of the imperial court. By the time of Cao Cao, Marten, Ma Chao and Han Sui had set out to fight against Cao Cao, but in the end, Cao Cao won. So strictly speaking, the Qiang people's settlements, that is, the areas shown on the map, should also be regarded as the territory of Wei. It's just that this is a plateau, sparsely populated, and there is almost no agricultural production there. If you build a city and then stay in the army for a long time, then the rations need to be shipped from Yongzhou or Bingzhou. The journey is long and the loss is huge. Compared with the benefits gained by occupying here, it is not worth investing so much military power and materials, so there is no city there. It is equivalent to giving up.