Because during combat with them, Zhou Tian astonishingly revealed his incredible life force at that moment. Considering their previous multitude of attacks, it was not only supposed to kill beings of equal standing to them, but after sustaining so many hits, even beings no weaker than them would likely have been critically injured and on the brink of death by then.
However, despite the seemingly powerful attacks that fell upon Zhou Tian, their final outcome only left the three living beings feeling disappointed.
Although their attacks did have an effect, once all those attacks were inflicted upon Zhou Tian, even though he was injured, Zhou Tian recovered in an extremely short period. Thus, based on Zhou Tian's performance at that time, there was no doubt that even if his strength might be comparable to their initial expectations, his life force, or rather, his self-preservation ability, was undoubtedly several times stronger than what they had anticipated.