With a mouse you can use the whole of your hand/wrist to articulate, with a console you are only using your thumbs.

Physiologicaly it makes more sense to use larger muscles for larger movements (wrist) while the smaller muscles (fingers) do the fine honing work as the larger muscle moves. In this case you wrist would be moving you towards the target while your fingers perform the fine micro adjustments to get a head shot. When using your thumb you have to perform every range of movement through the same muscles. I'm no kinesiologist, but everything I learned from sports/coaching would indicate the mouse is a physically superior control scheme for aiming.