The open source community is essentially a huge collaborative composition of people with various skills and interests that drive the results in a direction that is essentially a function of all those participants.
So, if you are some clever blogger who points out that the documentation is lacking for a certain group of people then the reason for it is obvious. None of the active participating components are people who care about the type of documentation.
The fundamental problem with this style of production is that only the manufacturers will be consistently pleased with the results. Today many people are interested in the software but unable or just unwilling to participate in its creation. That includes documentation of course. So until they are able to participate somehow, their interests will rarely, if ever, be represented.
In a way this is where commercial entities could really benefit this system. A commercial entity has interests beyond their own. In fact, in most cases their interests for the production are entirely outside their personal interests. A commercial entity that wants to rely on, say, KMail for their mail client in some one-off OS based on Linux may have a customer-base that is largely non-technical. Perhaps they are selling network kiosks to elderly or something. They will be particularly interested in proper documentation or help systems that appeal to those highly uninitiated.
But what happens with those actual real commercial entities with real needs for these types of missing components? It seems that they have a tendency to branch and the work they do that would benefit the average consumer of this software never ends up back in the main lines. Maybe because the mainline maintainers don't care, don't like it, or maybe because of licensing issues or perhaps... perhaps nobody gave it any thought yet.
At any rate, it still boils down to the same thing. The a classic "OSS" community developed project will generally only have features that are desired by the contributors. If you're lucky you'll have some contributors that seek to look out for others' interests but that seems to be incredibly rare in this subculture.
If maintainers of software cited for lacking this kind of documentation care about this issue, they should be proactive about it. There is an entire class of concerns that will rarely be raised by the sort of person able and willing to contribute to an OSS project. These concerns include aspects of UI design that benefit less technically savvy individuals and, of course, user friendly documentation. If the maintainers want to excel in the production of their software they need to reach out for these types of features. Find people who can provide the materials but don't know or want to know the processes involved in making the contributions themselves. Find commercial entities that have already done the work and try to integrate what they produce, or ask them to do it.