“Can't you get it through your thick skull?”
“You're so hard-headed!
Both are true of humans, but especially of male humans.
If you believe in evolution (“Don't monkey around with this one.”), then you might believe a recent report which indicates that humans developed thicker jaws and foreheads after they were able to curl their fists.
A curled fist was able to bash another face with much greater force.
Monkeys only did a lot of slapping.
To have more protection for their faces from the fists of others, stronger and thicker bone structure evolved:
“...fisticuffs spawned our facial structure, a phenomenon that's even more pronounced in men.”
Of course, a lot of damaging blows resulted until evolution could do its thing.
But “For the past 2 million years, the face of Homo Sapiens has gradually become less robust...”
It's good that boxing gloves and protective head gear also evolved.
IT'S ABOUT RENEWABLE RESOURCES!