Since the old gods could not directly force the God vein and take its resources, they could only go around this by putting their soul imprints on the beings created by this world. They watch over these people and watch them cultivate and accumulate energy. Once they reach a certain point and the host dies, they consume its soul and refine the accumulated power to help boost their own just like Damon was doing but at a much larger scale.
But the laws of the world encourage reincarnation but whatever these people are doing is causing imbalance, destroying the natural cycle of life.