"Nagpas can't learn from that which isn't destroyed. Imagine how bewildering and terrifying a city full of life must be to them. is it any wonder that they seek the ruin of such places?"
When Jim Henson designed the skeksis, he "became intrigued with the idea of a reptilian race assuming control over a formerly splendid past society." This eventually turned into the greedy, prideful, pathetically terrifying skeksis, which in turn likely went on to inspire the design of D&D's nagpa one year after the movie's release. With their latest incarnation featured in Mordenkainen's Tome of Foes, here's my take on their lore and mindset.
The nagpas were once a cabal of mages that sought to siphon power from the gods. As punishment for their greed and treachery against those that first gave them their power, they were cursed with vulture-like forms, blackened bones, and the inability to obtain knowledge from that which is not destroyed or discarded.
Nagpa's cannot learn of new things. At best, they have anterograde amnesia, able to recall fragments of new memories. At worst, their minds can crumble at the inability to comprehend what is around them; a nagpa that finds itself in a new city will shriek in fear, suffering short term madness until they spend sufficient time away from the settlement. The same is said for tomes on new magic or lore; to them, the modern mundane is the same as staring into an eldritch void.