Comment by hinkley

A keyboard is also a 3D interface with one dimension constrained. I have to be able to move my hand past the keyboard without accidentally inputting data. You can see the problems of this kind when you have a large touch screen and people try to gesture at it while talking and end up accidentally changing something.

Look at machine rooms in submarines or control rooms for power plants or tv studios. Different tasks are in different locations sure, but the inputs are constrained to an area near the arc defined by the fingers at a comfortable distance from the shoulder. For really good reasons.