Why not build a spaceship in the novel The Wandering Earth?

Because setting the level of science and technology does not guarantee to build a spaceship that can accommodate the world's population and replace the earth as the permanent home of mankind.

If a spaceship is built and dropped directly on the earth, and the earth is swallowed up by helium flash, there is no livable planet behind it, or even enough mass can be added (considering that the mass in the universe is very thin and the acquisition ability is insufficient), there is no turning back. But if we drive the earth away directly, we just need to find a stable star, position ourselves in the right orbit, and then restore its rotation. Such a star is much easier than finding a livable planet.

According to the novel on energy, there is a reunion technique, which just burns stones.