Take a toddler, put him/her on the beach and let them play with the sand...using their little hands they will let the sand slide through and what is it they create? A pyramid.
A clever grown up only has to look at it and can easily improve on it.
No alien involved.
Come on man, be real. I did say I believe the answer of who built the pyramids is only a component of this unexplained mystery (no pun intended).
Seriously you got to make better comparisons than that, playing with sand is well within the realms of physical capability, and is not complex, even the spasticated could make an attempt at this.
Building large land-fixtures is a little more challenging don't you think?