The gods are in constant conflict. So, if Pharasma goes to the Mosnastery of Veils (aka Anaphexia nowadays), who are now Norgorber's worshipers, Norgorber could also make a move towards Pharasma to help his newly acquired followers.
This agreement was concluded at the dawn of time, under a contract developed by Asmodée. While their followers are fighting all the time (as seen in the cemetery and the Styx River), they are not allowed to meddle with such mortals. The details of this contract are unclear, but it is related to the fact that Asmodeus helps other gods to imprison Rovagug.
Similarly, if the Pharasma psychopomps destroyed all the undead, we would have no undead. But there are other gods with an interest in protecting the undead, including Urgathoa, among others. As such, there is no way for psychopomps to deal with all the problems of the undead or anti-Pharasmin, because they have to fight against other outsiders who also want to cancel their work.
To learn more about Anaphexia, you'll find out more about the Occult Mysteries campaign settings book, but keep in mind that everything the pharasmins kept hidden in their vaults is now protected by Anaphexia. They did not just kill the monks and move on, they took their place keeping the secret that they hid, as indicated:
In this way, Anaphexia has worked for centuries, gathering secrets not only to use the divine obsession of his god, but also to pursue the work of the founders of the Pharasmin Monastery, while by preserving this wisdom with confidence for an age or mysterious event to come, and for which the modern world is not prepared.
Keep in mind that their clerics are powerful enough to launch Regenerate (Cleric 7), as indicated in their ability to restore the languages of their members. As such, dealing with most psychopomps is a trivial matter, and the most powerful ones (like Yamaraj) are never sent to the material plane.
Also note that they pretend to be monks of Pharasma, their identity as Norgorber's followers is not publicly known.
The Norgorberite killers of Anaphexia present themselves as ascetic monks of Pharasma and roam the world killing and flying to accumulate a multitude of secrets known only to themselves.