First, the life span is short, although the emperor prayed in his heart to live forever and rule the world forever. But looking at the dead emperor on their heads, we also know that this idea is unrealistic. Moreover, the ancient medical technology was backward, and the ancients all died earlier, usually in their thirties, and their lives almost ended. Especially those emperors who were busy and overworked died in their twenties. The emperor didn't want to have no place to live after death, otherwise it would be very bleak, so he would save for a rainy day and build a cemetery for himself first to settle his heart.
The second is the fear of the unknown. No one knows what will happen next, even the emperor has great troubles. They can't control the future. Even if they had supreme power before their death, everything would cease to exist after their death. The emperor also hopes to live in a beautiful house after his death, and he doesn't know how future generations will build their own tombs. It's better to do it yourself and enjoy the power in your hand first. There are also many emperors who don't build tombs when they are in power, but even the bodies are stuffed into other people's graves after death. It's so sad.
Third, Feng Shui Theory Even if tombstones are built in the countryside today, Feng Shui masters should be invited to see the location of the cemetery. In ancient times, feng shui was even more important, and the cemetery even affected the future fortune of a family. Especially for the purpose of the emperor, we must have Long Mai, surrounded by immortals, military forces and even all kinds of things, and it must be a land of geomantic omen. Only in this way can we keep the roots of the country and protect our great rivers and mountains.