Documentation is a form of API design, but where the code is prose and the tech stack is the human cognitive apparatus. Every person has a different one of those.
Imagine writing JavaScript where every single user has a bespoke browser.
Why is writing prose so much harder than writing code? 😂
Seriously, so much of the work of releasing code is the non-code stuff, like writing docs, blog posts, tweets, etc.
Even in code, communicating clearly is often the hardest part, e.g. naming things, API design, etc.