The secret spirits and humans in the Earth Secret Realm are merely beasts used for experiments by the Sky Secret Spirits.
Just like humans keep venomous snakes and fierce beasts for captivity and experimentation, humans also fear being harmed by these venomous beasts, but it doesn't mean humans lack the ability to subdue these beasts.
Or perhaps what humans fear the most is not the power of these venomous snakes and beasts, but rather the drugs humans experiment with on these creatures.
If a virus-infected experimental subject escapes from the lab, everyone aware of the incident will first feel fear, followed by attempts to stop the spread of the virus, with the best method naturally being to kill the virus-carrying subject.
Things might not be exactly the same, but the logic remains the same.
The cleansing of humans and Earth Secret Spirits is merely the Sky Secret Spirits clearing out failed subjects to make space for a new round of experiments.
