It was done by wealthy, well organized societies, just like todays skyscrapers. Things like great monuments are at the top of "pyramid". First you must take care about basic stuff like food, or security. And they are under constant threat in the world after collapse (IIRC Slavers are raiding those tirbal shitheads). They thought would be cool to underline their <s>tribal</s> primitive status with temples (where shamans can perform bloody rituals, and drink blood of sacrificed kiddies), voodoo magic, holy latex pajamas, spirits, grumpy bones, visions and stone heads. Spears weren't enough. They went full cliche, for the lulz.