There's not really a reason for a built in text editor

Is not a built in text editor, is a command to open programmatically your default text editor to edit a file (new or existing).

There are several cases when you would want to do that, for example, think on the git CLI, that opens one so you can write a commit message, etc.

And is not only about using the EDITOR variable, most non-programmers doesn’t have one defined or it points to vim or something equally terrible for non technical users.

You missed the point of what the library does. Similar to webbrowser (which isn't a full browser), all it does is open your default text editor.