why do people have to judge

why do people feel the need to judge someone by there looks alone?
its not fair. Just because I wear jeans with nice shoes, the occasional t-shirt, maybe a hoody, doesn't mean I don't take my job seriously.
the way I dress matches my personality, I take pride in what I do, i try not to be an embarresment, if I find that I need to dress in nice slacks, a blouse, and nice shoes then I will. Why can't people take someone abilities over the way they dress? I'm a pretty tolerant person but this is rediculous!