So, what gets me about Pern is the degree to which Pern forgets ALL THE IMPORTANT STUFF. I don't mean just by a little bit, I mean they forget by a whole lot.
Let's begin with origin stories. Humans love and adore origin stories. We like to know where things come from. These are so important to our culture that our very idea of who were are derives from origin stories. Americans remember that George Washington led the Continental army against the British. The Brits remember that the lords forced King John to sign the Magna Carta. The Christian world remembers that Jesus got arrested on disturbing the peace charges 2,000 years ago, sentenced by Pontius Pilot, and died on a cross. The Jews remember the reign of King David 3,000 years ago.
What's my point. People make a huge effort to keep and preserve their origin stories.
Now, let's look at dragon riders. What weyrling wouldn't hear the stories of the first riders and how they came to be? This is group cohesion 101. You tell a story that links you all together. The important heroes of yesterday would be remembered. The important stories would be written down, not just once, but many times over.
What do they do on Pern? They forget because it's easy to forget. Anne went on to invent reasons why they forgot, but in the end, those reasons are pretty contrived. Their Harpers have certainly done a terrible job of preserving basic information. I don't mean the stuff that can get burned in a fire, but the stuff that everyone gets a copy of, like a Bible or a Torah.
As for history, what is the first lesson that any child should know? It would be "our ancestors came to Pern because ..." That is the ultimate origin story. And why would they know that? Because that the most important lesson to learn. It's like knowing that Jesus died on the cross. Even atheists know alot about Jesus. Even the most apathetic would know the sacred story of Pern's founding because that's the important story that binds Pernese culture. The story would never go, "thread falls." No, the story would go, "Mankind came to Pern from another star to find a simpler life. The three ships were named <>, and we can still see them today, there on the horizon, the Dawn Sisters."
The average person might not know much about the first dragonriders, but the dragonriders would certainly know. This is really basic stuff. These are your heroes of yore. These are the exciting tales that get told again and again. The more that people get put under stress, the more that they need their cohesion stories. This is their Aeneid.
Likewise, each great hold would preserve the story of it's own founding. Same reason. People like and need their origin stories. It helps their world make sense.