It simply didn't matter even if the cultivator was a legendary Divine Ruler master in the hidden world. The fear of retaliation of Cain's backing is akin to a hanging guillotine.
The hope behind a Divine Mortal wasn't something to lightly crossed.
Quite literally, it is engraved into the hearts of people of the massive future and life-changing benefits just one Divine Mortal can give.
Martial cultivators who ascended into Divinehood gradually evolve their mental state. Their astuteness rises, allowing many to make more rational decisions that fall align with their Martial Nature.
Nobody in their right mind would ruin a long-term chance with a Divine Mortal for mere short-term benefits.
And behind the status of being a Divine Mortal was the unfathomable Shi Wei. Her deterrence became much stronger after Cain revealed his Divine Mortal status.