If I understand your question correctly, you are asking why we don't have more graves from the roughly 3,500 years of Ancient Egyptian civilization. The answer to this question is actually applicable to many aspects of archaeology, including (but not limited to) why we don't have more clothing, more wood carvings, or more food items: preservation bias.

All of the items I just mentioned, including human bodies, decay quickly due to natural processes. Even bones won't preserve unless special circumstances allow for it. Think of all the animal bodies you'd expect to find walking through the forest, but have you ever even seen them? How about just the bones? If you have, I bet it was only once or twice, because these things decay or are consumed very quickly.

There are more aspects to preservation bias than just organic decay.

Graves are something that help to preserve human remains, as they remove bodies away from predators and the elements. Burying a body in a grave is called inhumation, and a special form of Egyptian inhumation was mummification. The thing is, not everybody got to be buried, and certainly most were not mummified. If a body is exposed to the elements, or dumped in a river, it is gone very quickly. Even if the body is buried, simple graves were repurposed frequently (i.e. To make room for a new burial, people would dig up an old burial and dispose of the body). This was the reality for the vast majority of Ancient Egyptians (as well as ancient peoples in general). In fact, modern practice in many places outside of North America (including Britain and Continental Europe) is to relocate bodies that have been buried for more than a century or two so that the grave can be reused.

Finally, even if you are lucky enough to be buried and/or mummified in an elaborate grave, there is still the very real possibility of grave robbing. Your tomb might be 2,000 years old, but someone broke in 1,500 years ago to steal your grave goods and, in the process, dumped your body exposing it to the elements. Add to this the fact that building materials are expensive, and an elaborate building sitting empty for 1,500 years will very likely get dismantled for reuse. As an aside, this is why the discovery of Tutankhamun's tomb was such a big deal, as it still had a single chamber intact from antiquity.

TL/DR Burial isn't as common as you might think, with many bodies being exposed to the elements over the millenia. These bodies will not preserve. Some people did receive simple burials, but these were very often reused, which exposed the previous occupant and would lead to them not being preserved. Even tombs weren't a guarantee of preservation, as they could be looted, which frequently resulted in the body being exposed and decaying. The empty tomb would then become a target for reuse, as building materials were expensive.