Aside from the other factors already mentioned, also keep in mind refuse removal in ancient cities was poorly/not enforced, so that the level of the streets act

Aside from the other factors already mentioned, also keep in mind refuse removal in ancient cities was poorly/not enforced, so that the level of the streets actually rose over the centuries. The result being that if you tore down and old building to build a new one, the natural place to put the ground floor would be inches or even feet above the old floor, leaving the foundations of the old building in place. Places like Troy, which had millenia of habitation, basically ended up ontop of artificial hills (tels) of the garbage of previous generations.