Han people have developed civilization, why do they move south?

Han people have their own civilization, mainly farming civilization, and stick to their homeland. After the Tang Dynasty, the value of literature once surpassed that of martial arts. Although the people's life and culture are rich and the Central Plains civilization has reached a new peak, they lack sufficient strength to protect their own civilization. Just as the splendid civilization of ancient Rome was conquered by the Huns with less developed culture. Because there is not enough force, the Central Plains near the northern border is often conquered by brave nomads in the north. The nomadic people in the north are proud of being brave and good at fighting, and think that it is a sign of cowardice for the people in the Central Plains to move but fail to protect their land and food. On the one hand, the Central Plains people think that the nomadic people in the north are uncivilized barbarians who can only rob and do nothing. On the other hand, because the imperial court values literature over martial arts, they can't protect them, so they have to move south to escape the war.

In short, a developed civilization may not be able to protect itself, on the contrary, it may be vulnerable to disasters. Because developed civilizations have created rich living conditions, and powerful civilizations want to rob these powerful capitals. So I'm thinking, does a developed civilization have no effective power to protect itself?