1. Fear of your own death First of all, kings are afraid of their own death, because everyone is afraid of death, not to mention a king with a high position. The emperor is a man who stands at the peak of power. He gets what he wants, but life alone is beyond his control. When he thinks that he will not enjoy the present prosperity after his death, some kings will think that they will regret not seeing their descendants, so they are afraid of their own death and will try as long as they have a way to live for a long time.
2. Kings who are afraid of losing their power and status are afraid of losing their power and status because they have paid a lot to get it. Even people who are born into a prince will encounter many stab in the back when they grow up. Even if they don't have brothers, they will help their parents' brothers keep an eye on this position, so they are afraid of losing power after death and will not give up their efforts to fight for this position.
3. If you look at them better, you trust them. Many kings don't eat elixir in the first place. These things are gradual. At first, he might just drink a bowl of soup cooked by a Taoist priest. Later, when I saw that he was in good health, I believed this Taoist priest, and I would eat it as long as I gave it to the king.
When the status reaches the highest point, what you want to pursue is immortality. Some kings live very smoothly, the world is at peace, and famous ministers follow them. At this time, he has stood on the top of the world and there is nothing to pursue. What he can pursue is immortality that no one can achieve, so he went back to get the elixir of life.