The dictionary defines bullies as follows:
Now I disagree strongly with the term "those who are weaker" - many bullied people are not weaker. They are very often strong, talented and great people. You do not bully someone only because they are weaker, you get bullies who bully someone simply because they believe they themselves are stronger and better and they get a false sense of accomplishment and a huge ego boost from this.
Many bullied people don't fight back. There are a number of reasons for this. If this is physical bullying they may desist either out of fear or because they are no match. Mental or emotional bullying however is different. A bullied person may simple step back because of fear, misplaced respect or in many many cases concern for what the outcome or retaliation or consequence may be for standing up to the bully.
So if you are being bullied - find someone you trust and speak up. And if you are witnessing bullying, stand up and speak up.
c'est la vie