Because we also have up to 9th level spells. As soon as the world discovered that people were capable of the work that could be done with 9th level magic... every nation would spend every gold piece to get their best and the brightest to increase productivity with that level of magical technology. Do you really think after 10,000 years of magical research and development in the Forgotten Realms... creating magic on par with what 9th level spells could create and do (especially with elves who could actually live throughout half of that)... that their world would still be at a medieval level of construction?
Heck... look at where we are as a society after 10,000 years of advancement, and we haven't had 9th level magic available to us that entire time! They have in the Realms. Their Industrial Revolution should have happened millenia ago. But because we just like knights in shining armor, they still use wagons to get around and have walled cities you need to throw ballista bolts at.