There are many reasons why these would not be reported. Although sometimes the reason may be:
1.) When the sexual abuse (SA) can be inflicted by a family member or close family friend, this can lead to 2.) the victim being scared to tell adults or the authorities. Although sometimes, the reason the victim is scared (if it's a child) to report it because the parent is the inflictor.
3.) Often times, when SA is inflicted by a sibling, parent, or family friend, the victim is afraid to report it because they may believe it's their fault it's happening to them (they're fault they're sexually abused) or they may feel weird because of it.
4.) When Gender-based violence (GBV) occurs, it's usually between children (teens mostly) at school or outside of safe places such as home. The reason this is not reported because many times adults/authorities (such as teachers) don't take it seriously and the kids feel neglected because of this. This then leads to the child (when they're affected by the GBV in the future,) believing that no one cares and or it's not important and or wrong. Therefore, not reporting the act because of one neglectful adult.
“This could use more explanation/length.”