Perhaps because the man with the RPG is taking a hell of a chance. Won't the rebels, in this case, need to get within range first of all?
Perhaps you should think what war is meant to be about.
The simple rule for users of RPG's is 'don't miss with your first shot', which means being close. If a chap isn't sufficiently motivated by his cause to take his (quite reasonable, 90%+ first round hit probability) chance against a moving T-72 at 100m, why they hell are you zooming around the sky doing his work for him?
War by proxy rarely brings about lasting change. You'd have thought the west would have realised this by now.