The reason why is that the PWHL is simply Intermediate hockey.......with the age as you described.
The fact that the Leagues has only about 15% of true age players is a result of the OWHA rescue attempt to have this age group continue at the AA level.
However if this league wants to be considered Jr hockey then the age group is correct.....the adjustments that have been described many times or the majority of them need to take place first.
Play hard, play fair
The Diceman