What do we mean by 'documentation'? Usage of the outwardmost-exported interface (which, depending on the project, may be an UI or an API)? Configuration and customization options, installation instructions? Design rationale, deep-dive into internals, explanation of internal components?

'Documentation' covers a wide swath of prose, and therefore makes for an easy target for criticism which is in fact very disparate, despite appearing homogeneous.

That is not to say most projects have good documentation. Writing prose -- specifically, technical writing -- is a skill orthogonal to software development and doesn't necessarily tend to attract the same kinds of individuals.