If you look at it all from a Biological point of view, we Humans are animals and thereby have animalistic traits. And in the animal kingdom, the aggressive and big animals are the ones that are dominant (Excluding Humans who are dominant because of technology). Now, a female animal will always try and chose the biggest and most aggressive mate because it will increase the chances that their offspring will survive since the male is dominant over all the other males. Now, apply that same logic to Humans. Girls are "shallow" because they want (They're programmed by their genes to want) the tallest and most aggressive mates (boys) so that their offspring will be protected from other competition etc.
I think this makes sense. Girls don't want the weak guys with nice personalities because their offspring won't have guaranteed survival. Although, society doesn't involve men killing other men just to get with a female and so almost all Human offspring aren't at risk of getting killed by male competition. But still, girls have been programmed to want the biggest mates because we are still animals at the end of the day.
tl;dr : Girls are shallow because they've been biologically programmed to want a male who can protect their offspring