Knowledge is passed on from master to apprentice. Nearly-universal literacy helped to alleviate this issue somewhat, but even today most practical knowledge is passed on by people working together.

The result is that even a gap of a single generation is enough for some knowledge to be lost. That's why Roman concrete was forgotten, why we can't build a Saturn V anymore, why you can't just start a chip-fab with enough money, and why it's so hard to maintain code without access to someone that designed the code-base.

Most things, even if they're written down, are more accurately modelled in someone's brain.