The spiritual energy of the world was similar to all living things. It has a cycle.
Akin to that of a ball thrown into the air, the most powerful person should be born at the peak of the spiritual energy cycle. Then, upon its decline, the spiritual energy of the world would sharply decrease in quality. It would then become increasingly difficult for anyone to make a breakthrough in their cultivation.
This represented the end of an era. Even if one had terrifying strength, they would still be unable to display it. They could only seal themselves up and wait for the next spiritual energy recovery which signaled the start of an era.
A cycle represented the coming and the end of a life.
A life for ordinary people was about a hundred years.
A tree's life could last for thousands of years.
Plantlife only lasted a single autumn.
Different lives had their own different life cycles.
This was how the world worked.