Why does everyone think in stereotypes?

I'm aware that stereotypes are there for a reason, but you miss out getting to know really great people when you generalize like that. Not all females are these annoying, emotional, uptight, whiny, and shallow things that a lot of men make them out to be. And not all males are these brainless doofs that only care about sports, beer, and getting laid that they have come to be known as. I just find this whole gender studies section to be really annoying. What happened to being an individual?