Here’s how it could happen, if you read this article, even he makes reference to historical periods during which the level of workmanship had declined from earlier periods.
Many great empires and cities lie beneath the dust. For some period of time Rome was being used to pasture sheep.
It’ is illogical when you have a culture hellbent on creating their own very very fine artistic works yet the older works are even more advanced? Did they all of a sudden “forget how” to create the previous much finer works? They decided to go backwards in skills and pride of craftsmanship rather than advance?