>why is writing things down better?

For the same reason we make kids write out math problems rather than just reciting them. It engages different parts of the brain for one, you gain an expanded amount of context by doing so.

The next thing has to do with human memory, when you remember something it is never read only. Recalling memory can adjust your neurons and change the memory, sometimes a little, sometimes a lot. Hence why eyewitness testimony is mostly trash.

Writing it down also provides a larger workspace for making changes that you can loop over. Quite often things sound fine in our mind, then we write it, and reread it and we realize its wrong. These days people commonly use this in agentic workflows. One agent will conceptualize an idea, and then it's passed off to another agent with a clean context to review for logical flaws. Reading ones own text does the same things for humans, we don't load the full memory context of what we wrote and it seemingly allows us to iterate over the problem differently.

And I guess lastly, human voice is very low bandwidth. A wagon full of well written books is far easier to manage than the humans that contain that knowledge. The reader is at their leisure to learn and the speaker isn't inconvenienced by being at someones beck and call.