[In such a short time, you brought them hope and despair over and over again. Your presence alone was hope for them, but at the same time, your disappearance has become despair. The power you possess symbolized their hope to survive so they become nervous when you leave.]
The God of Hope doesn’t stop explaining.
The reason is longer than I thought.
[They can only live on my mercy in a world polluted by your poisonous fog. They will never return to their hometown and will be trapped in the Holy Land forever. Even in such a dire situation, they will not lose hope for the future. It will eventually pass and their safety in the Holy Land is guaranteed. That’s what I like. You brought them new despair when they were ready to start anew. Because of that, they will never lose hope.]
*From The Tutorial is Too Hard.
This is basically what Leo-san is talking about this whole time. His existence purpose is to save humanity not to keep humanity safe, there is a minuscule difference that leads to this. If Leo-san can't save humanity because there is nothing threatening humanity then he can't fulfill his purpose.
Leo-san needs humanity to be constantly in danger so he can save humanity over and over.