Main Preparations.
The position of Celestial Supreme has been taken away since the divine planes fused, but the Authority of the Celestial Supreme can be acquired again with enough contributions. This will also grant their possessors the right to create a pantheon.
What's more, there is no limit to the number of Celestial Supremes anymore. Any Celestial can become one. They don't need to suffer under their previous Celestial Supreme anymore.
That's just one of the many benefits that gods stand to gain from the era of conquest. But the gods don't need them to participate in the upcoming battle because there is a threat of death for them.
They may be gods, but they feel like mortals in the face of a war that concerns the fate of two realm trees. Not only is the battlefield their home, which they have to defend, but if the defense fails and the realm is destroyed, they too will go with it. So the benefits are the icing on the cake for them.