This was because after reaching the Shattering Realm, not only could one shatter the external appearances, but they could also shatter themselves, causing their body and soul to be between a shattered and complete state at any time, as if they were between reality and illusion.
This way, unless one broke the boundary between reality and illusion, it was impossible to kill a Shattering realm expert. To do such a thing, one needed to have far stronger powers than the Shattering power. As for what realm it required, Lu Qingzhu herself was not very clear.
Moreover, even if the boundary between reality and illusion was really broken and her body and soul were shattered, she could still leave an indelible mark between reality and illusion.
As long as this mark still existed, she could reform her body and soul over the long years and fully revive again.