In this case, I'm fairly certain your last sentence gets to the heart of the issue. The biggest motive of most people writing OSS stuff is: Fun. Documenting is

In this case, I'm fairly certain your last sentence gets to the heart of the issue. The biggest motive of most people writing OSS stuff is: Fun. Documenting is un-fun. Since there is 0 obligation on the author's part and any benefit gained by others is a side-effect, documents won't get written, particularly to appease people who come in an demand better docs without offering any compensation.