Actually an arrow to the face is possible to block, so is a stab to the torso. But a horse can not be blocked, you can possibly survive with all your bones crushed underneath it. Or perhaps you can run away, or dodge it. Anyways it is damned hard to avoid a horse when you are at the front lines, and 99/100 times the ones in the front is really killed by a horse charge. However a arrow rain is much easier since you can actually just hide behind your shield. And a melee combat you can just stab the enemy before he stab you. If you try to block the horse you die. If you stab the horse it will fall over you and you die. If you try to run away the shields behind you will push you back and you die. If you actually survive a horse charge then you can be damned since then comes the slaughter of the scattered forces. Cavalry is not to be taken lightly, a cavalry charge back in those times should be just as frightening as a tank rush on you position should be these days.
One day, someone showed me a glass of water that was half full. And he said, "Is it half full or half empty?" So I drank the water. No more problem. ~Alexander Jodorowsky