what do you think happened?
An older civilization built all of it and Egyptians just moved in (same with the Mayans).
You should check out the book
Fingerprints of the Gods. You can find it
here.
The author's website is
here.
I read the book. Its not totally convincing but at least the author has some cool stories to tell. One is about bribing guards and climbing to the top of the great pyramid at night to watch the sun rise.
There's a lot to cover in the book, and a lot of his claims are rejected.
About the sphinx though, and I don't remember exactly, but the point that it faces on the horizon lines up with where the constelation leo crossed it in the past. The thing is that leo didn't cross there since thousands of years before the egyptians, around the time when it last was rainy there.
I can tell you more about what I remember from the book if you want me to.