Now i'm not saying that striking is ineffective. But we have to come back to the fact that it plays a very small part in the majority of sources.
The main reason in my opinion is that if 1 - you can strike then strike them with a weapon. 2 - If you don't have a weapon smash them in to the ground control them and get or draw a weapon and strkike them with it. 3 - they have a weapon and you don't. Then control them smash in to the ground and take their weapon or go to 2 and repeat if necessary.
Explaining why there is so little striking with the hand is of course the problem. They didn't do it. There has to be a reason. They were the ones who fought in armor, not us, so we must assume that they had a good reason. It does not get us near the answer or any degree of understanding to say that punches are great and effective and whatever. The old masters did not seem to think this was the case.
One thing is clear, that wrestling/grappling has two benefits. First, it nullifies the enemy's ability to strike with a hand weapon, or even his hand. Second, it provides solutions for what to do in the clinch, after you've tied up the guy's hands.
The throw is intended to put the man on the ground in, hopefully, a position in which he is in a disadvantage, a position where you can finish him. Landing on on the head, or otherwise incapacitating the throwee is, in fact, rare. In most throws/takedowns, the guy does not land on his head, contrary to popular modern belief. He has fight left while on the ground. But it is more likely in the old days that the fight would have been concluded with daggers than with strikes or submissions.
Here is an account of a judicial duel that occurred in France in 1386.
A dagger thrust decided the last formal judicial duel fought in France in 1386. Jean de Carrouges accused Jacques le Gris of raping his wife, Marguerite. He demanded and ultimately received permission for trial by combat. The fight began with a joust, but without any fence separating the combatants. They made two passed without breaking a lance, but on the third pass, both lances shattered. The fight continued on horseback with axes. During the wheeling melee, le Gris struck a great, two-handed blow at Carrouges. Carrouges raised his shield in time to protect himself, but the axe-head deflected from the shield and struck Carrouges’ horse in the neck, killing it. Carrouges leaped clear as the horse collapsed. Le Gris charged, trying to impale Carrouges on the top spike of his axe. Carrouges leaped out of the way and thrust his own axe spike into le Gris’ horse just behind the girth. Carrouges’ axe head sank deeply and stuck fast in the wound, and the horse’s momentum tore it from his grasp. Le Gris’ horse then crumpled, and the fight continued on foot with swords, “thrusting and striking and slashing.” At last, the two men nearly spent, le Gris stabbed Carrouges in the thigh. Le Gris drew his sword from the wound and stepped back as if expecting Carrouges to fall. But Carrouges grasped le Gris by the top of his helmet, and pulling sharply forward, threw le Gris to the ground. Carrouges stood over le Gris and tried to stab through the fallen man’s armor with his sword, but was unable to do so. Finally, when le Gris tried to strike from his fallen position, Carrouges knocked the sword from his hands and fell upon him. Kneeling on le Gris’ chest, Carrouges continued to thrust with the point of his sword, but with no more success than he had had before. Reversing the blade and pounding le Gris about the head with the pommel of his sword, Carrouges succeeded in breaking the latch securing le Gris’ helmet visor. Carrouges flipped up the visor, and cast his sword aside because he could not hold le Gris’ helmet and wield the sword at the same time. Carrouges drew his dagger. He demanded le Gris confess the rape. Le Gris cried that he was innocent. “Then be you damned,” shouted Carrouges, and drove the point of his dagger under le Gris’ jaw and into his brain.
The answer thus seems to be that men in the middle ages and renaissance thought that punching was less effective in combat than relying on the dagger.