Being from an era so long ago, little is known of the Milak Shivaar, not even how they went extinct tens of millions of years ago. But it's believed that's not why they've been elusive. The Madek Shivar seemed capable of living above and below water; and not inside enclosures the way the Zeborno preferred, but they seemed to have methods of breathing underwater as well; and so it's likely that many of their cities are deep in the oceans.
Even above ground, the Milak Shivar preferred high levels of humidity, typically living in hot and damp environments everywhere they lived.
They worshipped gods of war, and probably became the dominant species of their time because of it. However, despite this, they do not seem to have gone extinct as a result of violence.