Why did the Chinese build powerful defensive walls only in the north?

In fact, this thesis is false. The Chinese built powerful defensive structures from all sides. But the most powerful were built on the north side. The reason for such actions was the fear of plunder by the nomadic peoples. By that time, the Chinese had accumulated enough knowledge and money to become the coveted prey of the Hunnu people and others. The structures in western China were supposed to protect trade routes from raids and looting. The defenses in the eastern part reached modern North Korea.