Habit & Fool
Even though there was something at the bottom of the cave neither Michael nor the Elemental Empress wanted to leave the cave. For one, the cave was in his territory, so he had to deal with potential issues sooner or later, either way. Other than that, Michael had already constructed the Underground Forging Hall in the cavern.
The Temple of the Forgotten was also in the cave. Even his miners could mine day in and day out in the cave since the humongous cavern system had various ore deposits. Meanwhile, the Elemental Empress' decision was mostly based on her gut feeling. She felt that it was dangerous to move deeper into the cave through the large cavern tunnel, but she could also tell that nothing bad would happen. If anything, the pressure spreading through the entire cavern system was something that could aid her society to grow stronger and more resilient.