Why doesnt the military make helicopters with protective armour around all the rotors?

Well even now, helicopter rotors are already bullet resistant against small arms (M4's, Ak-47's, M16's, etc.) But no matter what you add on to any vehicle, armor piercing rounds will without a doubt cause too much damage to protect from. Not only that, but if you add the rotors on the edge of the rotors, then the flow of air created from the rotor would be disrupted and would make it difficult to move from side to side. And rotors are incredibly hard to hit as you can't really hit it without hitting the bottom of the the helicopter to begin with.