First, because those in the God Realm have already "made it ashore"; as long as they don't enter the Space-Time River that contains the Path of Cause and Effect, they can greatly delay the advent of tribulation.
Second, Lan Chuan Divine General and the others have stayed in the Primordial Realm for a long time. In order to explore higher Realms, they've already adapted to the Primordial Realm's Origin. If they remain outside for too long, they themselves are easily affected.
At the same time they would also affect others, for the Origin of the Desolate Domain Era does not allow too many beings of the God Realm to walk the world.
True Gods of No Tribulation fare slightly better; but if a Free God stays in one place for too long, the Origin will begin gathering upon the Free God, and everyone else will not only gain nothing, but even suffer tribulation.
