Humans are relatively conformist, and we often have a hard time translating abstract/revolutionary ideas in to practice. It seems likely that many humans had ideas for things resembling civilization, or things that could've lead to the development of a civilization, before the first actual civilization, in the same way more people dream about starting businesses than actually start businesses.

Paul Graham seems to think that local culture plays a huge role in startup success. Now consider that even the cultures Paul Graham considers pretty bad are still American city cultures, and America has a reputation for individualism, rags-to-riches success, etc. and that's all on a foundation of enlightenment values related to progress, questioning authority, and so on. And we've got a long and storied history of society changing on a large scale, within our lifetimes even.

So yeah, stagnant cultures are not necessarily being held back by lack of intelligence. It could be the standard akrasia/agency-failure type stuff that we're still struggling with today. (Arguably something similar is going on for peoples' failure to appreciate the possible magnitude of human-level AGI--it's just way too bizarre relative to historical norms for most of us to take it seriously.)