Others seem to disagree. I suspect it has to do with investment in certain aspects of D&D lore.
Perhaps these setting-specific fans want completionism for one-true-wayism, rather than content that helps other DMs?
Truly, setting-specific proper nouns belong in a setting guide, where it helps those who use the setting. These proper nouns do well to be absent from a monster book, where DMs who use other settings might use as a resource.
But oblique allusions to someone else somewhere else is useless for the DMs who need a description of a creature, in the first place.
Proper Nouns are distracting. The DM stops thinking about the scene and starts grappling with some random NPC in some random planar location somewhere else. The best case scenario is if this irrelevant NPC exists. Worse. These random NPCs and their random cosmological locations probably dont exist in other settings, so the description leaves the DM with no description at all.
Reliance on proper nouns also reduces the value of the monster book for other settings.
It makes little sense to pay money for setting content that the DM plans to ignore in the first place.
Why pay money to rewrite everything?
The monster book is a more cost-effective investment for other settings when its description focuses on the scene at hand - how do these monsters work well together? - and ignores proper nouns that may or may not exist in other settings.
A description of the creature - as opposed to the description of some random NPC - gives info that the DM can use to describe an encounter. Who cares if (insert powerful unique creature here) did it? The players looking at the scene will never know this. And even if they did, who cares? Proper nouns are useless and fail to give the DM the tools that the DM needs to describe an encounter.