Why did/do sites such as the pyramids in Egypt or the Roman colosseum end up in an abandoned state, only to be "rediscovered" later?

As I understand it - which is not at all - the pyramid complex in Giza was always next to a bustling inhabited city, but the complex itself seemingly went ignored/untouched for centuries. Same goes for famous Roman sites. Why were these objects and sites not reused or maintained or destroyed until relatively recently? Where did everyone go, and why weren't they living in and around these structures this whole time? And if they were, why didn't they do anything with the sites?

I understand that empires and civilisations come to an end, but they aren't the result of wholesale genocide, and even if they were, the genociders would surely move into that area next and continue living in the pre-built cities and towns. But that doesn't seem to be what happened.

Why is humanity out of the picture in these monumental and impressive sites for unbroken periods of deep time?

Cheers!