It's easy to see how traces of an advanced civilization can disappear (with some exceptions) over time, as the most destructive force in nature is not always nature, it is sometimes mankind itself. The reason only one Antikythera Mechanism has been found has a lot more to do with the value of metal than the scarcity of technology, as any non-functional metal devices left over during a barbaric period wouldn't have been preserved as is, they would have been used to make other items. Ditto girders and structural metal. Heck, the skin of the Great Pyramid was damaged in recorded history and there are almost no traces of any of it after locals used the stone for construction.

The things that last are stone items (non-valuable stone only, none of the Mica found in South and Central American ruins survived modern-day scavengers, for example) and constructions too large or massive to destroy. Anything made of anything valuable would have been melted down for scrap long ago.