Rome was invaded many times since the forum was built; the Celts invaded in 387BC thus earning the title "Founders of Europe". Centuries later the Barbarians (Ostrogoths, Vandals etc) plundered Rome, so perhaps ruined buildngs were used as footings or foundations for what followed. The Renaissance Romans would not have had the equipment to do anything else than build on top of what was already there ...(hope one of the many people with 'dig' knowledge comes back on this one).

I have only been to Rome once and it is an astonishing city of the ancient mixed in the with modern. It was fascinating that one of the bridges we drove over was a couple of thousand years old.

http://www.aviewoncities.com/rome/forumromanum.htm

Many things can cause the build up of soil/anything covering a building - sometimes it's not a build up - and in ancient cities, many suburbs get abandoned for hundreds of years, so who'd notice?

It's a great question.

You get instances like Dublin where the Viking Thing Mound was leveled to raise the level of one street by some 16 foot or so. A great loss to the City!

I think it's in York where you can go below ground and see the first floor level of houses.

In a lot of cases it's just easier to rob away most of a building and cover it over in order to start afresh, so town planners just raise the level of the land to create a clean sheet.

Obviously, this isn't the case everywhere. Winchester is a great example. Apparently, the Romanised villas were abandoned there and people returned to round houses built amongst them. It must have looked odd with loads of stone-built ruins in an orderly-street-plan fashion with the huts set amongst them. Eventually, people moved on and settled in new areas leaving the ground to swallow up the remains.