(Apologies if this has been asked before - a search yielded little direct info.)

Seems like there are often stories of excavations in Rome or wherever uncovering previous "layers" of the city, where structures were abandoned, somehow left partly or mostly intact and later people just...built over the top.

Why did this happen? How did this process even work? Why weren't these buildings just knocked down and salvaged or summat? Where'd all the dirt come from to fill the gaps?

Or am I totally misunderstanding what I've read?