Is there a Watsonian reason for Westeros being stuck in medieval stasis for several milennia now? Or is it just George being beholden to some very traditional fantasy tropes?

Oh no, you said the s-word…

This is one of my major pet peeves in ASOIAF fandom. I’ve talked about it here and here and here and here, but I’ll repeat myself here: Westeros is not in medieval stasis. 

Rather, what we have is a story of technological change from the Paleolithic through the Neolithic through the Bronze Age through the Iron Age through the Early, High, and Late Middle Ages, with the Free Cities sitting firmly in the Renaissance. 

The reason why we think there’s medieval stasis is that Westerosi historians, much like medieval chroniclers, have re-interpreted pre-medieval history in medieval terms, so that warriors become knights, chieftains become kings, and gods become heroes.