I was discussing this graph with my cousin the other day:

He observed how preposterous it was that we don't know for sure how the Pyramids were built but we bloody well know the interest rates of Babylon.

I countered that 95% of the documents he had stored for safe keeping were receipts, warranties, tax returns and contracts, and why on earth would we suspect it's ever been different? I made a ritual of burning my homework and notes at the end of every quarter and the only books I kept were the ones I couldn't sell. And yet, because grown-ups tell you to, for years I had utility bills going back half a dozen years in case I needed to establish my address or some shit.

The documents that get preserved are the documents that keep us from getting screwed over, or prove to someone else that they aren't being screwed. Much like money, literacy and math are most used to interact where there's no trust. We know about Gilgamesh because Ashurbanipal decided to aggrandize himself; we know how much beer workers got in 3000 BC because if they didn't get it nothing got built.

Survivorship bias is a very real thing and the more advanced the culture, the more their relics favor H&R Block.