Not to be pedantic, but that is not simply "a piece of wire with two connectors"
If you look at the number of contact pins on the camera's hot shoe and the bottom of the SB-400 foot, you'll see 5 contact points. The modern camera's "hot shoe" is not just an on-off switch - it's really a data port for the camera to communicate with the speed light. The camera sends data about it's current settings like F-stop, shutter speed, zoom focal length and ISO info to the speed light which use the info to determine the amount of power to engage when it fires the flash of light.
Just want to make sure that speedlight sync cables get some respect around here