Yep simple answer is lightly framed developing bodies that by natural design are not yet capable of coping with such an amount of for high impact, explosive, hard hitting contact training and rigour.
Only problem is scar tissue formation which never heals 100%, ala chronic hammies and calves ect.
The boys require close management as they develop into bigger stronger men, as well as considerable management as they age if such injuries become a problem.
The growth of the pelvis in males is often an issue as the male pelvis does not tilt fwd on an angle unlike that of the female thus exerting a more stable less flexible degree of muscle and ligamental load than with females.
Thus can exert extra loading and pressure upon the ligaments and muscles in the lower back, hips and groins, primarily abdo, and adductor muscles, with hip flexors to a much lesser extent in the growing underdeveloped male, which is not an issue unless undertaking such physical strains and stressors as is the case at AFL level.