It's easy to be captivated by the medieval world of Westeros in Game of Thrones. I mean, everybody loves a good story about kings and queens, princes and princesses, swords and lords and castles and banners. But have you ever wondered why Westeros is stuck in this never ending medieval age? They have lived like this for thousands and thousands of years. How come their society hasn't progressed? Nerdwriter blames the dragons. It's an interesting point of view that makes a lot of sense (even if it's applying our history and pace of progress to a fictional history and world that includes all kinds of weird magic). In the middle ages of our world, we had castles that could withstand sieges just like in Game of Thrones. But eventually, cannons were developed and it no longer made any sense for lords to wait out war in castles, because they would get destroyed by the onslaught of cannonballs.
Instead, they had to wage war in the open field, which meant they needed to wield larger armies to give themselves a better chance at winning, which meant they needed more money, which meant they needed more taxes, which meant they needed a better government system. A government system that would progress to our nation-states today. Gone is the castle system. Bye bye, medieval period.
In Game of Thrones, they don't have cannons. They have dragons. And you can't just raise taxes to buy dragons. And if you don't have a dragon, you can't just raise a bigger army to defeat a dragon. You pledge fealty to the people who have dragons. You keep yourself in the same system because of the power of dragons.
There's more to it, too. The whole video explaining how dragons stop progress by Nerdwriter below is really, really fun to geek out on.