From what little I have pondered on this I always assumed the ancients built huge monoliths because of the earth quakes and severe conditions of the earth. The small stuff would collapse so they built big - over engineered the things. Maybe the reason this "first" civilization is thought to have outposts all over the earth was because they were monitoring the heavens for signs of an approaching cataclysm. Humans don't build things just to build them.
http://www.halexandria.org/dward197.htm
http://www.velikovsky.info/Immanuel_Velikovsky
http://www.immanuelvelikovsky.com/
“Humans don’t build things just to build them.”
Actually, I would say that is a uniquely human quality. We’re the only species on the planet that DOES build things just to build them, when they don’t have any practical purpose. We call it “art”.
Perhaps the ruling class discovered early on, that if you keep them occupied and tuckered out, the peasants are less revolting.