Why do certain breeds of dogs (Pit Bull, Chow, Rottweiler, German Shepherd, etc) have such a bad reputation?

Because aggression is a natural tendency in some breeds. I have 2 dogs that are a mix of wolf, king sheppard and akita. I've had them since they were puppies (they are 9 and 10 years old now) and despite the fact they were brought up in a loving home, they still can be aggressive. Although they're aggression is directed toward other animals, I still would not trust them around my babies. They do not get near my kids because I'm not willing to roll the dice and take a chance on them harming my babies. It's not stereotyping them, it's that most dogs like this are aggressive by nature. I don't care how ''sweet'' you think your pit bull is. In a second, he has the capability to KILL your child. Having him around your kids, supervised or not, is playing a game of russian roulette.