Why do people sometimes think that bullies are tough and confident?

That is just a stereotype. Usually, when there is intimidation or an excessive use of force, it is the person acting out of fear or overcompensation for their insecurities. They often are outwardly tough, but inside, they often feel inadequate. Sometimes they are overweight, and they are teased about their weight, and they lash out using their fists or hurling insults.

Plus bullying is not always one-dimensional and things are not always as they seem. Often, the bullies are victims of bullying or abuse elsewhere.