Why the Three-body Man escaped and the Earth Man could not go extinct?

The Three-body Man was an alien intelligent life form that had lived on Earth for a period of time before escaping for some reason. Due to their high intelligence and powerful technology, they could use various methods to escape the pursuit and attack of the Earthlings. Even if the Trisola people managed to escape the pursuit of the Earthlings, they could not guarantee that they would not be counterattacked by the Earthlings. Earth was rich in resources and energy, including all kinds of materials and energy needed by humans. Therefore, even if the Three-Body People escaped from Earth, they could not rule out the possibility of extinction. In the novel, the Three-Body Man escaped from the Earth and the humans could not go extinct. This plot was to ensure the development of the story and also to reflect the wisdom and courage of the Three-Body Man.