Official documentations are not always complete. It depends on the diligence of who wrote them and how good they are at writing. Customers and users will always send mails or open tickets to ask this and that about the docs afterwards. Can't rely on just learning or retrieving from the docs. Clarifications by some dev or someone who found a solution/workaround will always be required.

If people are withdrawing from writing secondary documentation, why wouldn't they also withdraw from writing official documentation?

And don't say they wouldn't, we've already watched business decisions be made to pull back on spending to produce official documentation as the availability of the internet grew to the point vendors understood that everyone has always-online internet connections.