There are a lot of people who just aren't that eager to have other people know they've been raped, sexually assaulted, demeaned, or whatever. Something bad happens to someone, for many their first reaction isn't to tell others; it's to keep it to themselves and make sure nobody ever finds out. Also, there's probably a perception - probably even a correct perception - that reporting someone, especially an important someone, for such acts in an organization doesn't tend to lead towards the guilty person being punished and can actually damage the accusers reputation. A lot of times, there's little incentive to report the problem. That may be changing.
But, even if you don't connect with those ideas and can't fathom why a person wouldn't immediately report such an event, I'm sure you understand that people, in fact, don't report many of these events. You realize that's how many people are and the choices they make. Rather than looking at this and trying to determine how they "should" react, I think it's more helpful to sit back and just observe how people actually do react in reality, understand their hesitations, and think about how things can improve.