Why do northerners in China always attack Zhongyuan people?

Nomads invaded the Central Plains in order to plunder wealth. What is there to plunder when the Central Plains invades the northern grasslands? The northern grassland area has harsh climate, low productivity, unsuitability for agriculture, vast territory and sparse population, and is not rich in products. Only those cattle and sheep (compared with the huge population and wealth of the Central Plains) really have nothing to plunder. It is meaningless in terms of the needs of immigrants. It is precisely because of the harsh climate and low productivity in the northern grassland areas that there is great pressure to feed nomadic people, not to mention large-scale migration.

On the other hand, it can be analyzed from the cost. Central plains people are not as good at fighting as nomadic people themselves, so it is very expensive to train at least one army that can defeat nomadic people. If we want to station troops on the grassland (which is a necessary guarantee for occupation), the annual cost will be even greater. Some people can say that the occupation of grasslands can not be for plunder, but to eliminate the hidden danger of future nomadic invasion. This cannot be denied to be correct. But in fact, in the history of China, except the Qing Dynasty, no dynasty had enough national strength to bear the expenses of stationing troops in the northern grasslands (or even in the western regions) for many years. Therefore, occupation was an unattainable goal for a long time (including the heyday of Han and Tang Dynasties).