I think the reason is that you can get quite much of a tunnel vision while developing. Everyone involved already knows what they're building and how it looks, so it's very easy to overlook it.

Case in point, I recently wrote a CLI to show which of your AWS infra is not captured in terraform, and you can either print the result as a json to consume by a machine or generate a dashboard with some charts. It took me an embarrassing while to realise that I should have included a damned screenshot in the README, in fact I think I only realised when I wanted to show it to my brother.