So there are a few reasons. One, either by natural disaster or war the structures get destroyed and just not rebuilt. Two, getting building materials was difficult and expensive. So it was easier to just take existing materials from an old unused building to use in your new building. And finally, when a civilization collapses, the vast majority of people leave the city. So you have a massive city with no one living in it. You just don’t have the population or tax base to maintain all those buildings. For example, in 200AD Rome had a population of 1.5 million. By 500AD it had collapsed to just 100k. It didn’t get back to that 1.5 million until the mid 20th century
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