You're also missing something. Skulls aren't actually rigid. Bone is not rigid. Bones have some elasticity to them. They are also porous. A thick and strong skull is not going to pass on blunt trauma more than a thin one, because bones are literally made to absorb impact. Thus a thicker skull is always a better protection than a thin one.
Charges are literally impact damage. Maybe in games the charging person doesn't receive damage, but in reality, both the attacker and the defender receive the same amount of force. In other words...any animal that charges as a habit will be virtually impossible to defeat with blunt trauma to the head by anything that it can charge at, because it not being hurt by its own charge means that it being attacked in return will also not do a thing.
So yes. Punching it in the head will lead to far lesser damage than attacking behind the skull.
I'm not saying that the main character can't defeat it by punching it in the head, by the way. I'm just saying that he would be able to defeat the pig with far less force if he attacked somewhere else.
