Because it just works (tm). And it is flexible to a point that no GUI can ever accomplish. It's liberating. It's repeatable, It's automatable. It's about control. And most importantly, it's FAST!
If you try to max out the control, GUI comes out of as an UX disaster. Check any enterprise software GUI to see what I mean. There will be lot's and lot's of buttons all around, and you would also end up with some kind of text input or programming environment inside it.