Because they want to stand around and giggle whenever a guy that they deem attractive walks by.
I used to work in a jewlery store in a mall so I've observed this every day for about a year
Atleast I was nicely dressed so I could give my ego a boost when now and then the random group would walk by and one would awkwardly wave and then the entire group would burst into giggles :hehehe:
Strange beasts these teenage girls.