It's based on the fact that we don't know.

We've seen multiple times in archeology when we've originally thought of places as "failed state" or "collapsed" it's actually turned out to be a conscious rejection of that way of life. Most famously Cahokia. Which at one point was a larger city than any city in Europe. We now know that it didn't "collapse" so much as there was a political revolution and people decided to move towards different ways of life.

It's best not to assume failure when you have no evidence for it.