oh i think that's easy
my theory is that -
the capstone was placed on top but stayed there by its own weight, not affixed to the pyramid,
and that when The Flood (of which Noah survived) came, it swept the capstone off and swooshed it away somewhere
I think the pyramids were built BEFORE The Flood, by giants who died in The Flood.
The capstones are probably laying in an ocean somewhere, or maybe instead, covered in sand someplace just like the Sphinx used to be.