they lack the muscles to move their shoulder musculature in the proper way. this is from sedentary lifestyle of sitting and repetitive constrained movements along limited narrow planes of motion. they're like circus tigers who've been tied to a wooden post in the ground their entire lives and as a result their hind legs have atrophied to the point where they can't even jump.
it's abhorrent that most educated privileged modern men/women, even those who are "fit" and attend intense work out routines 3x/week and think they have a kickass "bod," are little more than stunted animals with dysfunctional bodies.