There's surely a lot more to be said, especially about particular sites like this that are in urban areas, but some of the answers in the threads referenced by the links in the top comment in this thread by /u/jachooltiger--though the answers themselves are by many others--may offer some preliminary insights. It is rather stunning how quickly sediment can build up in the right conditions, without humans continually clearing a site, but of course someone specializing in recent urban archaeology may be able to speak to the particular examples you're talking about (shorter time scales and denser settlement). Still, I found those threads interesting for raising the basic ideas, some of them touching on more recent ruined sites.