Why the hell do girls wear short shorts?

I've had this bottled up for years now, because it's embarring to tell my parents or sisters about it. But my sisters are 14 and 15, and they always walk around the house wearing foot long pajama shorts. Everytime i see it, i get so pissed off, and i also feel envious of them. I'm a 17 year old boy. I always call them a ****** slut in my head for the way they dress with their tank tops and ****** shorts. I just want someone to understand how i feel. I don't understand why they have to dress like that? Can you understand why i can't tell my parents or them about it? It makes me so envious how a girl can show her legs like that, but i can't. My body is something to feel ashamed of, so i have to hide it up. I ****** hate it when they bring their stupid girlfriends over because of how ****** prissy they act. They wear short shorts too.