I've just admitted what few will. I follow stereotypes almost religiously. Day in and day out I label new people I see like animals, cells, or a strain of the flu, not as actual humans. I use terms like goths, emos, skaters, preps, "gagstas", sluts, and more every day. I would consider carrying a descriptive encyclopedia around with me and study the subjects like a field scientist.
Why? Look around you. Our world is defined by stereotypes. Everything is owned by Pepsi and Coke. It's not cool to be yourself and be different these days. Instead, you're expected to ignore your uniqueness and fall into a random clique that most likely is not even close to who you really are. Teenagers are even encouraged to abandon their true beings and jump on a bandwagon.
Call me heartless, call me inaccurate, call me lazy and uneducated, it doesn't matter. Sure, hiding beneath designer clothes, Hot Topic makeup, and certain music there are people that fit well into the category or group but yet their persona shines. Do I have time to go around and check to see if every teen I see isn't completely brainwashed and mindless by mass media? No.
tl;dr - Kids these days follow stereotypes because they think it's the right thing to do, stereotyping them is easy and most of the time quite accurate.