Chinese take the centuries long view, and the NVA was supported by China. That’s the answer. The Viet Nam war was never going to be over, just like the Korean war is not over. US doesn’t fight with a long view. We need things settled instantly. Our people are instant gratification types.
Simplistically that’s the Western Way of War** - maneuver & fight toward a climatic battle that settles the issue. The Eastern way is raid avoid raid avoid raid avoid raid until the enemy goes away. Does the Eastern way seem primitive & tribal in nature. The answer is yes!
** See Victor Davis Hanson’s Book - The Western Way of War