Why don't people stop when they see an accident?

My daughter was behind a vehicle, and it was swaying. So she called 911, as she was talking to the officer, the vehicle lost control. My daughter got a small child out, he was probably 8 or 9 years old. She couldn't get the driver out. He was covered in blood. She stayed with him, until the police arrived. She was frustrated, because alot of people drove by, but no-one stopped to help. Why don't people stop, when they see an accident?