Lol, I could ask the same thing about guys. People want to fit in, and it's lame to judge them for it, especially when most of us are guilty of doing the same thing at one point or another. Perhaps you're only seeing what you want to see, or only what's on the surface. I don't know a single person without something unique about them, whether it's in mannerism, style, personality, etc.
Um... I don't want to be like everyone else, thank you very much. That's why I don't straighten my hair, I don't go out partying and drinking every night, I've only ever worn heels once, I actaully really LIKE hanging out with my mom and little brothers, I LOVE old movies (like from the 40's and the 60's), I don't wear skinny jeans, My favorite actor is Jack Lemmon, my favorite singer is Frank Sinatra, and my favorite movie is The Odd Couple (by the way, watch it if you haven't yet - It's the funniest movie I've ever seen!)... is that enough yet? :-)
So I shouldn't act like everyone else? How should I act? What should I wear? hahah(:
On a serious note, the girls you are referring to (not all) are probably just insecure and want to be viewed normally or looked upon the right way. Maybe someone bullied them before so they're afraid to be different.