In the past few centuries, the Immortal Realm experienced events like the Reversal of the Dao, the Split of Daluo, the Ritual of Blazing Flame, the Expedition to the Wisdom Realm, the Ascension of Taiyi, and the Descent of Evil Gods. Most peak Golden Immortals have been through countless battles, and those who survived, after a period of settling, have an even better chance of finding the path to Immortal Monarch.
Although this ratio is extremely small, it's offset by the large base number of peak Golden Immortals in the Immortal Realm.
Therefore, the births of Immortal Monarchs tend to erupt in clusters.
Han Yi's other guess was that the Extremely Ancient World sensed an impending calamity and differentiated fates to select peak Golden Immortals, thereby catalyzing more Immortal Monarchs to strengthen the power of the Extremely Ancient World.