Ask two people from the same profession what something is and they'll give different answers.
Domain knowledge doesn't exist in a vacuum — it's worked into our entire life experience, building upon not only related concepts but also how our imaginations work, our worldview and our biases.
Everybody has their own mental model of how something works — which is itself a small appendage of their own mental model of everything.
How should we write documents if everyone understands things in a different way? (Telephone Game) Is it an effective use of our time, given that the systems we build are always changing? (Hitting a moving target) And is it a good use of our energy? (Writing is hard)