This is because the behavior is ingrained in biology. But make no mistake behavior ingrained in biology is from selection pressure and that selection pressure is sexual selection. Men select the women who are more interested in making themselves look pretty.
>There is communication too between girls via appearance. boys don’t enter the picture until much later.
The communication is this: If a girl looks prettier she is saying, "I'm better at attracting and manipulating men than you." That's the only thing communicated.
>They communicate with looks.
You're thinking too much. Do men communicate by getting rough and rowdy? Do men communicate by playing sports? Maybe? But the extent of the communication is minimal and it's the same thing with women and looks. There's hidden communication network that men don't know about, that's much to imaginative. Additionally, if such a network existed, some woman would eventually spill the beans.