1. People have to learn the basics of the GUI. Take someone who lived 50 years without seeing a computer and show them Windows. They will have to learn that they can move the mouse, left/right click, and even move while clicking. They will have to learn that some parts of the UI are interactive (e.g. buttons) and others aren't. It is not as intuitive as people tend to imply. My father still double-clicks on elements that - to me - obviously don't require a double click (like a hyperlink). And many people are completely lost if you move them from their Windows to macOS or some Linux UI. Or even between two flavours of Android.

2. Once you know the basic of GUI ("there is a start menu", "I can click on buttons", ...), the UI does not show you that there exists apps like Photoshop to edit pictures, or like Slack to talk to people. You have to somehow learn about the app from somewhere, install it, and only then you get their UI that "shows you what's possible".

I would argue that the CLI is actually quite similar. Once you have learned the basics ("I can change directory", "I can `man cmd`"), then you have to discover apps somehow (e.g. a friend saying "install git with `apt install git`"), and then you can just open the manual with `man git`, and it will most definitely show you what's possible.

The difference, I think, is that people generally don't really want to learn the CLI (it "looks old", I guess?), and many apps don't provide proper manpages (pretty sure most developers don't know how to even create manpages, or package their app for their distro).