Reasons behind why average fish only live for 3 weeks after sold

For some fish, it's merely days after they're sold, if that! I'm referring to goldfish sold at funfairs or carnivals in plastic bags. They must be treated terribly. Poor kids who win one of these after paying to play a game might feel happy at first, but then when the little fish dies after a couple of days it can be quite saddening. I was one of those kids. But obviously, I myself must have made a few newb mistakes, although my fish bought from a pet shop (also goldfish) lasted much longer.