Not enough time to counter RPGs
A RPG (rocket propelled grenade) only has a flight time of a few seconds, depending on the range. The favored tactic against helicopters is to lure them close to the ground then fire a bunch of RPGs, which are unguided but very cheap. A single hit will take out a helicopter (RPGs are anti-tank weapons). If you can get within 100 meters or so of the target, the flight time would probably be only 1-2 seconds.