I want you to imagine for a second that you are living within the walls of Rome in 600AD. Your once million plus city has fallen to less than 50,000, and you decide one day you want to expand your home. Do you go find a quarry to get the stone? In a manner of speaking. You walk to the Circus Maximum, the Colosseum, the Forum, and you take what you want.

The majority of buildings in the City that survive do so because they were repurposed and/or protected. Look at the Colosseum for example; while the one side exterior collapsed in an earthquake, most of the damage was done by people taking stone and other materials from the site. The parts that collapsed were reused. This mirrors the fates of many other buildings in the city. Some were damaged by earthquakes and natural processes, but many were simply razed slowly as well (so, a combination of both factors). The buildings that remain in the best shape were those that were protected by the Church, either as fortifications (such as Hadrian's tomb), churches (like the Pantheon), or for other reasons (such as the Colosseum, which was ostensibly saved because of the role it played in the martyrdom of early saints).