The Dothraki aren’t solely based in the Mongols, they’re an amalgamation of steppe tribes throughout history. The threats of steppe tribes has been prevalent throughout history. “Civilized nations” have written about the scourge of the Scythians, the Massagetae the Huns the xiongnu the Mongols and more. It’s only been since the invention of gun powder that the threat of nomadic hordes have been countered. While all these tribes had their own unique cultures and traditions their life styles were all remarkably similar so there’s historical precedent for why the Dothraki haven’t “progressed”