I constantly see those projects that would benefit from a screenshot in a readme, but for some reason the thought about adding one doesn't even come to the authors' mind, even though it's pretty obvious. I'd really like to know what's going on.
I think in many cases the authors value functionality much more then esthetics.
Which is a bummer because it is a complete different point of view from the more typical user experience
"A picture is worth a thousand words"
It's like we've had this ancient knowledge passed down for generations but people constantly just ignore it.
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.