[Redbob]

""To build straight walls up 8 meters ... I don't know how to do it today without mechanical equipment,"

I can certainly imagine how it could be done;
build an earthen ramp up to the height the stone needs to go, drag it up the ramp on rollers (round logs), building the ramp higher for each succeeding course of stones.

When done, haul off the dirt from the ramp.

Simple, not easy.

[Snurple]

I agree. Simple answer...whoda thunk it. People love a mystery that’s why they constantly come up with new ideas on how the pyramids were built.