Moving to disarm forces the shooter to worry about maintaining control of the gun over shooting at a target. If you are close enough, you can likely move quickly enough to engage the shooter without getting shot.
On the other hand, moving for your firearm, and doing so in front of a shooter who is covering you with his gun, only forces the shooter to make the decision to shoot. Heck, I would be worried that any movements might result in getting shot. That's why you take your chance with a quick move at the shooter and engage in hand to hand combat if you are close enough to do so.