So, I remember watching this made for TV mini-series about Queen Cleopatra. There was this scene where she and Marc Antony were marveling at the Pyramids and the Sphinx. Marc Antony asked her to tell him the story behind them. Cleopatra said that they were so old that no one really knew which pharaohs built them or for what purpose, though there were legends.
That always struck me as odd. But at the same time, strangely plausible. Considering that Cleopatra lived closer to our time than to the Old Kingdom, it wouldn't be surprising if most of the details about the Pyramids had been lost by then. Also, I'd imagine that modern archeologists with the ability to read hieroglyphs and current findings might actually know more details about that period than someone in 1st century BC Egypt.
Edited for typos.
Do Egyptologists know more about Old Kingdom Egypt than the common Egyptian on the street in Cleopatra's time? Possibly. Only a very small percentage of people in ancient Egypt were literate, and most did not have access to temple libraries. Royalty and court scholars were literate, however, and they were familiar with the history of Egypt. Prior to the decipherment of hieroglyphs, Egyptian history was known primarily from historians in the Greco-Roman period like Manetho (3rd century BCE), to whom we owe the numbered system of dynasties that is - for better or worse - still used today.
To answer your question more directly, the Egyptians of Cleopatra's time had not forgotten the builders of the Giza pyramids. There is always more to be said on the topic, but I wrote about this in Is it true that "two hundred years after the last pyramid was reared in their country, Egyptians were as baffled by the big masonries as everyone else?" In other words, did they forget how to build the pyramids? How/why?