I think the problem is the vague use of the word "developer."
Some people develop programming languages. For them, a GUI-less environment may be ideal.
I do web development. So I need the GUI, not just for testing, but for dealing with image and video assets that come in from the art department, or creating mockups, or presentations, or maps, etc.
It's still "development," but unsuited for a 100% command line experience.
As is often the case, it's easy for someone in one field of "development" to forget that it's a broad category of experiences.