The history that survives is partly the history somebody had space to store.

What reaches the future is filtered by something ordinary: room. Letters survive because a family had a dry attic and did not have to move every two years. Records survive because an institution had shelves and a budget and nobody needing that space for something else. So the past we can read is tilted towards people who had stability, and the gaps are not evidence that nothing happened there.

When you read history, ask whose papers these were and who could afford to keep them. And in your own family, remember that what you save now becomes the record somebody later relies on.