Are you seriously asking, or is this one of those sarcastic rhetorical questions? The point of all of these studies is that society has deemed women shall be paid less. You asked about models, but it is also prevalent in acting as well. A leading female actor will be paid less than a male actor in a similar role. It gets tricky here regarding the perceived attraction/recognition to the audience, but it does seem to default to male actors being paid more as the default.

You can ask why till the cows come home, but there will never be a satisfactory answer. Just asking the question to imply wrongness does not really do anything though.