A few reasons unrelated to defense: economy of motion, plus your sight picture is already established before you reach extension. You don't draw, reach extension, then line your sights up. The other guy will have finished the stage in the amount of time you'll take to do that.
If you extend your arms and lift them to the target, it's likely you'll pass through your sight picture because you must brake your movement. Even if you brake on time perfectly, braking costs time and has to be avoided at all costs.
They teach it as 3 separate movements and you practice each in isolation to master each part. But once you put it together it is one fluid motion and it becomes automatic.