Question

Two reasons why many cases of abuse and violence are not reported

Answer 1

The kid is jumped and beat up from some bullies at school, he may be afraid that they will beat him again worse if he tells on them. Fear of retribution.

Answer 2

The abuser might warn the victim that he/she will harm the victim even more if he/she tells. They fear retaliation from their abuser.
Or they believe that no one will help them or can help them.