There are lots of reasons it goes un-reported. Shame, guilt, intimidation, public knowledge of having to deal with the stigma attached to it. Fear of retribution. Exclusion from activities you enjoy or are dedicated to.
Personal story. I'm an adult hetero male. 6'6" trained to take care of business if needed. I was a victin of sexual child abuse for years at the hands of my brother, his friends, later my scount leader and also a lifeguard. All male. I did NOTHING to deserve any of these and none of these were related to the other events.
All of the above reasons, were the reasons that I never said a word. My brother said he would kill me if I talked. I was 7. I believed him. Scout Leader said he'd throw me out of the Scouts. Lifegaurd said I'd never become a lifegaurd if I ever said anything.
We as males were told and taught we just tough things out and not show weakness. It was years ago. The only person I still would have any idea how to press charges on, would be my brother. My parents, don't need to endure that. No good can come out of it. I don't talk to him anyway. Haven't for years.
And that is the story of how these can happen and never get reported. As an adult now I feel badly that I did nothing to stop it as I'm sure it set a standard that allowed them to continue with others. But when your young, you process things differently.