The paths would be repetitive. Wouldn't it be great if, instead of drawing a new path in 3D space every single time someone files a flight plan, someone studied the area surrounding the airport, taking into account obstacles, traffic, the fact that there's a residential neighborhood on one end of the airport that shouldn't be bothered at night, and prevailing wind patterns, and drew all those 3D paths through the sky and published them for everyone to use, so that traffic follows known predictable paths?

And in the case of a deviation, would it be faster to pick up a stylus, draw a new path in 3D space, click send, and wait for a message acknowledging that... or someone yelling "27 alpha turn right heading 270 immediately"?