To be certain, we do not know exactly how old the Egyptian pyramids are, nor the Sphinx.

The only real evidence we have for Cufu being the builder/inhabitant of the great pyramid is a small grafitti carving inside one of the small rooms (constructed for stabilty) above the king's chamber. Beyond that we can only speculate.

The steppyramids are however of the date we assume them to be, and the writing on the walls is actually fairly 'primitive', fitting with their time of suspected construction. So, this lack of evidence might not really be anything.

But the Sphinx is indeed odd. There are simply so many things that do not add up.

You know guys... When I watched National Treasure, and they found it, I said to my GF "Screw the gold and jewels... give me the scrolls of the library in Alexandria." I consider the loss of that knowledge to be the greatest loss for our civlization. Even if we are just dealing with fictious tales and poetry (which I don't read)... The connection to our most early days, to our time of legends.
Who wouldn't like to read Ptolomy's account of Alexander's campaigns? Or the big manual for phalanxwarfare by Polybius? Or the collected works of Fabius Pictor?
We see references to these works in all our most important historical works. And some times they say that those works are what we should look up to get a proper picture of things, ect ect. I pains me to stumble over such mentions.