Yep last time I've checked the Roman aqueducts are still there and well the pyramids are also there (unless you think Stargate is an accurate historic account of events).
People weren't stupid, and building big whilst labor intensive isn't particularly hard.
We marvel at those aqueducts and pyramids in the same way we marvel at this. WE shouldn't pooh-pah one accomplishment just because another exists that we're culturally more familiar with.
It's plenty hard in an age before mechanization. You have to:
-source materials and have them delivered
-employ expert engineers and craftsmen
-manage the financial needs of a project without electronic communication
-ensure safety without the sophisticated techniques and supplies we take for granted in construction today (hard hats?)
It's worth noting that the great pyramid of Giza was actually the third attempt, after the first pyramid collapsed under construction and the second had to be "bent" to avoid collapse as well.
So I think it's fair to say that big building was actually quite difficult, even for seemingly simple projects.
People weren't stupid, and building big whilst labor intensive isn't particularly hard.