How have they not found gunpowder or invent a basic gun

Not the exact same reasons. ASOIAF has no gunpowder, while the high Middle Ages did have it. People did make use of cannons in sieges in this time period though since they did not have the ability to cast metal of the quality needed to make cannons out of a single piece of metal, medieval cannons were instead made of a bunch of pieces held together. They had a high failure rate, and frequently killed their crews, meaning that they did not find much use outside of siege lines. There is a few examples of hand-cannons (pretty much proto-guns that require a flame to shoot) from this time period, but they were not very common and suffered the same problems as cannons.

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