Why are girls physically weaker than boys?
They have a finer musculature, but mostly it is the skeletal structure. It, too, is finer, even if a woman is physically strong. Marine Corps Basic Training is very difficult for a woman to pass. The rigors of the effort more often than not lead to stress fractures, particularly in the pelvis.
Evolution. Men did of most of the physical labor, hunting, and fighting. They still do, really.
That's not always the case. It usually is the case though... just to do with hormones.
Less testosterone and muscle mass. ???? ????