This can be said about 99% of GitHub repositories.

When I first started using GitHub, I thought there must be some reason that no one ever included images in their README. Maybe it's not considered cool, I thought.

Visuals are hard to include because you need to store them somewhere. If you store the in the git repo, they bloat the repo. It's really annoying to clone a repo and having to wait forever because it has a bunch of binaries in it. I try to use SVG whenever possible, sometimes animated with CSS (see example[1]) as they are made if text and are much smaller. When I need non-SVG images, I host them externally, but that's a bit if an overhead and you need to make sure you don't break the image link accidentally.

[1] https://github.com/gotesttools/gotestfmt

I believe if you just copy paste an image in the GitHub text editor, or drag and drop one into it, GitHub will automatically upload it to their own image upload service and will insert the markdown code to show it.

I believe that's true, but I'd guess most people don't edit their README using the GitHub editor.

I believe that it's this: https://twitter.com/_nonfigurativ_/status/172732259457002734...

That is neat as hell!

Yeah, it's confusing to see empty READMEs like this, especially for a visual effect. The demo is at https://bgstaal.github.io/multipleWindow3dScene/ if you want to play around with it.