Let's pile up an enormous amount of rocks, which probably contain trace amounts of metals. Hey, look, they have a noticeable effect on magnetics. Can't explain that! It's always amusing when people talk about how a primitive people couldn't have done something like this. Why not? The Egyptians were fairly advanced, and this monument was of absolute importance to them and their gods. They probably spent thousands of man-hours calculating how and where to place this, and they had an entire country of people with nothing to do between planting and harvesting to haul and chisel stones. You get the world's largest ever public works project as a result.