The basic requirement for the assessment to ascend to the Fourth Palace is that a Martial Artist's realm must be at the level of a Seven-star Void God. This single requirement has stopped countless people in their tracks.
In the Qingmu Department of the Third Palace, the vast majority are Five-star or Six-star Void Gods. Some of these people have been stuck in the Third Palace for tens of thousands of years. They know that stepping into the level of a Seven-star Void God is incredibly difficult.
Even if Jing Yan's talent is unmatched, entering the realm of a Seven-star Void God is by no means an easy feat. Perhaps he would also spend several centuries or millennia in the Qingmu Department of the Third Palace. By then, even if he succeeds in ascending to the Fourth Palace, how much benefit can he bring to the Qingmu Department of the Third Palace? Moreover, even if he does bring some benefits, it's hard to say whether these benefits will fall to those people.