Surely, if nobody is using the software, then there’s no incentive to keep making it.

Making a tool you or the company you work for need yourself, fun, learning, community, doing good, showing off, status, being remembered, (even if it's just in a circle of 10 people)...

Marketing generates interest. Interest gets users. Users (hopefully) get donations and/or contributions to the project.

Irrelevant for the vast majority of open source projects, which will never be financially profitable.

why not be clear and avoid wasting people’s time as they try to figure out what exactly a project is about?

Maybe because the volunteers working on the project in their free time are programmers, not marketers or good communicators?
Also, they aren't wasting anybody's time by creating useful software and giving it away for free.

I realize I'm being confrontational towards you, but this mindset of demanding things from people who literally give away free stuff with no strings attached rubs me the wrong way, every single time. And this mindset is much too prevalent, even to the point of harassing, insulting and threatening open source devs for choices they make in their projects.

The devs owe you nothing. If you don't like what they do, simply don't use it.
There are other options out there, but they may come with a $23/month price tag.

You are right. I just checked out gimp.org, and...IS there a single image of the software on that site?

If they want new users, asking them to blindly download software without even a look or maybe a video of new features is not it.