Writing is hard. The scope, structure, and style of every written document differs for each topic, making it an unknown quantity every time. This makes it hard task to approach for the writer, and readers never know what to expect.
Making non-writers write long documents is always a gamble because they are probably not good writers. They would probably prefer to be doing what they're great at, and their teammates would probably prefer that too.
And once this hard work is done, the documented information will immediately begin to diverge from reality. (Hitting a Moving Target)