Reply #7: Very much right, but not wholly answering the question

[Kurt_and_Hunter]

Agricultural societies not only can have a population surplus, they must have a surplus. In ancient Egypt the intricate irrigation system would survive most annual floods but be destroyed by the really bad floods. And when destroyed the entire system had to be rebuilt fast or everyone would starve.

In the off years that surplus labor built giant stone things.

That explains how monuments were possible but doesn't explain why, of all conceivable uses of such armies of surplus labor, various sorts of epic burial mound were universal.

The Romans didn't go in for that sort of thing and used similar energies to build stone roads, stone aqueducts, etc. Such things would have economically advanced any civilization more than literally heaping rocks as high as they could go. (The Egyptian "bent pyramid" suggests that, like European cathedral builders, they may have even sometimes sought to stack rocks even higher than gravity was happy with.)

As with most things, an interplay of human nature and practical reality. Whenever we developed the overwhelming desire to be buried agriculture was nowhere on the horizon. (Cro-magnons buried their most cherished dead with artifacts and decorations.)

When agriculture came along suddenly our primitive innate desire was able to explode in scale and complexity.