Being preoccupied with your body and the way you look or Body image is very Real part of being a girl and is especially strong during early adolescence.
There are several reasons why they care about how they look but I'm going to only focus on a few.

Puberty is the most important and sensitive marker during this time causing changes in hormones, body, and how you think.

We make it look easy but being a female can be challenging. Girls are more likely to have a negative body image from media and an increase in body fat during puberty resulting in having a harder time adjusting to our bodies. Early physical development can add to the stress of being a teen girl.