Why do people judge you without knowing you?

Judging a book by its' cover is as old as mankind. How you look and act give people their first impression of you, and usually it's a lasting one. And you can say that your outward appearance isn't a reflection of who you really are, but why isn't it? You dress like a slut because you want people to look at you. You dress in black because you think you're goth. You wear your pants below your knees because you want people to know you're ghetto. People dress the way they do for a reason.