Because it's thankless. The bosses I've had generally seem to think that writing documentation beyond inline comments is a waste of time; something that shoul

Because it's thankless. The bosses I've had generally seem to think that writing documentation beyond inline comments is a waste of time; something that shouldn't be done unless scheduled for. And of course it's always the first job to be postponed if the schedule is tight (it always is.) At work, documentation gets written when the boss isn't looking. Corporate culture makes writing documentation taboo. The examples of great documentation I am most familiar with are written for open source projects where managers aren't around to hassle people for writing it. Particularly: mpv, ffmpeg, racket, emacs. In my career I have yet to encounter a commercial software project with documentation at this level.