As a counterpoint to 'way too many zeros, why haven't they advanced technologically yet?', I posit this, and in fact use it occasionally in my homebrew.

In real history, technology has risen and fallen as empires and nations have risen and fallen. Case in point: the Romans had the ability to mass-produce steel, the Greeks first came up with the Atomic Theory, and had Rome not declined, they may well have imported oriental gunpowder and made it widespread some thousand years before its eventual spread. Of course, we all know what happened to Rome, and the resulting Dark Age knocked civilization back to mere subsistence. The Black Plague wiped out two-thirds of the population of Europe.

Things like this happen. When a civilization starts to become stagnant, they tend to decay, and Bad Things start to happen, generally resulting in the fall of the culture, and going back to more primitive roots.

I will say, however, that precise dating for ancient artifacts is rather silly in most midevil settings.

"Those are ancient ruins."
"How old?"
"Who knows? They've been here as long as we have recorded history."

Oh, that does bring me to a pet peeve... Ancient ruins that have stood for thousands of years, and yet no one has plundered them before the party shows up? No signs that any other intruders have been there and taken some of the loot? This goes double for ones near significant populations. Some noble or another would have decided to go plunder it long ere the party got there, either for altruistic reasons (for the historical value) or selfish (loots!).

It's possible that there is some treasure in ancient ruins left, but it won't be obvious, and all of the readily accessible areas have already been searched dozens of times. You're going to need to get to somewhere deep within that no one has gotten to before. That's not going to be something that a group of level 1 people are going to manage.

Throw in some evidence that it has been ransacked many times before. Have 'secret' concealed passageways left lying open on the first few levels, rubble where walls were broken through, traps disabled.

I'm reminded of a scene in one of the Elemenster books where he was stocking a tomb with treasure for the next group of adventurers to follow. It made me laugh, because it pointed out how absurd that NO ONE would have searched ruins that are commonly known about.