If you think about it, pretty much nobody in the Star Boundary is giving a single shit about the Heaven Devouring Great Emperor. The other great emperors most certainly aren't trying to track him down and eliminate him. Hong Cheng was going around asking for help, but received none. And then the great emperors just proceeded to forget him. They couldn't care less even if they actively tried to. The only time they seemingly care is when making public appearances, to maintain their face. Otherwise they have better things to do, such as guarding their disciples/daughters so that the girls won't fall for Yang Kai. This after the great battle where a few of the great emperors fell, yet didn't even manage to kill a single Wu Kuang. They are all bona fide first-class dumbasses.