Aside from the fact that GRRM is writing it as such, is there a good in-universe explanation for why everything has basically been stuck in the medieval period for thousands of years? Even when considering the Andals and changes in weaponry, Valyrian engineering with the roads, etc., the world has been mired in an essentially stagnant state, developmentally-speaking, since the Dawn Age. With 12,000 years one might expect at least a little more technical, social, and material innovation.

Yeah, I don’t agree with this characterization at all. Westeros has gone from the Stone Age to the High Middle Ages in 12,000 years, and Essos is firmly in the Renaissance. 

Given that the Paleolithic Age lasted 2.5 million years, the Mesolithic 5-10,000 years, and the Neolithic anywhere from 6-8,000 years, it only looks like stagnation if you arrange your comparison to include the unprecedented development of the last few hundred years. There’s a reason they call it the hockey stick graph.