You know what? As has been pointed out in this thread, they have had thousands of years and made very little in terms of advancement. In fact, things such as forging valyerian steel have been lost. Only the most skilled of smiths can even reforge it. I think it is just best to accept this is a kingdom of idiots.
Well the Valyrian steel forging required dragons, and they had died off for quite a long time. They didn't necessarily forget, they physically couldn't.
And between magic, dragons, frozen undead armies, and what have you, this kind of ends up like a Dark Ages era. Technology and science stops making strides when you kill them all off as heretics or whatever. The warfare seemed to be the only thing where progress was made, which... is quite realistic. The way science and technology go, they make large strides in fits, some major discovery that feeds other discoveries, but sometimes there isn't that singular leap for generations.
Also don't forget about burning down and having to rebuild entire towns/kingdoms -- this likely means libraries go up in flame.
Recall that it seemed like some segments of the GOT world seemed to have made more advancements than others.
And lastly, recall there seem to be actual god(s) in this series - that or the magic is a little unpredictable? The God of Light may very well have held them back at various times.
While it wouldn't be the case here, it could be something like the Dune series, where specific technologies were outlawed after AI nearly wiped out humanity. Now I'm definitely curious whether GRRM ever put some backstory to this concept, whether as part of the main book series or in some side work.