Most Helpful Guy
They dont necessarily. Bad boys do tend to show more confidence though and confidence is what is really attractive here. Especially for 'good' girls, because 'good' girls tend to be more afraid to do the things 'bad' boys do, hence the seeming confidence bad boys show.
Most Helpful Girl
probs because they like how out-of-reach "bad boys" are. they might be too scared to try out the things that bad boys do, and thus admire them/take an interest in them because of that