For one thing, not all of it is. The various fora adjoining the main forum, and the main forum itself (the one in the valley between the Capitoline and the Palatine), are for a combination of reasons. One is that the city has been sacked and many of its buildings destroyed many, many times over history. Furthermore, the area is not necessarily known for this, but it is still occasionally seismically, active. The ancients – and really everyone until the modern ages – lacked industrial equipment to move rubble and debris, so often they just built over top of it whenever they could. Add to that the occasional deposition of soil from floods of the Tiber, which could definitely reach as far as the forum and did as late as 1898, and you’ll see the levels of the city begin to rise.