Firstly, the opposing side occupies such a vast world, and the real top-notch experts are not just clones stronger than Zhou Tian. It's likely that beings capable of fighting or even defeating Zhou Tian may exist in this world, but they are of high status and so would not easily encircle Zhou Tian like common soldiers.
However, if Zhou Tian were to be trapped in this current underground cave, in order to eliminate Zhou Tian as an enemy, those experts might come running at any moment. With their strength, if those beings were to take action against him, Zhou Tian wouldn't stand even the slightest chance of victory.
Thus, if those high-level experts indeed took action, Zhou Tian's advantage would vanish, and his safety could not be guaranteed by then.