I think all the reasons people have said are right. And I think there are others too. And the fact of the matter is that abuse makes everything complicated and confusing and scary for everyone. Suppose you report and it makes things WORSE? Suppose you report and nothing happens? Suppose you report and the abuser comes after you? What is the good ending supposed to be? When people never talked about this, it was quite literally unthinkable and in any case unspeakable. And no one had any stories or ideas how it would go after they tried to intervene. And when there was a huge amount of stigma imposed for being an abuse survivor, it was not at all clear that being publicly identified would be a benefit for the victim. Add in that abusers have an uncanny knack for choosing people to vicitimze who have little or no social support... The whole thing is just too depressing for words.
Things have changed in my lifetime, but not nearly enough. Abuse like what my H suffered would not go unreported now. But now there is a sliding scale, and the lines between what is ok and what is abuse are fuzzy, which is progress and the line for "unacceptable" has moved (or at least in some quarters it has...) And that doesn't actually make it less confusing. And when people are confused they tend to just gloss over and move on.
I think for a lot of people the paralysis comes in because they just can see what the outcome should be. If a parent is abusing a child... ideally you'd want the parent to "get better" and become a good person and a good parent and the family to get counseling and heal... and it seems absurd to even write it at some level. If it is a case of abuse outside a family I guess we want the perpetrator to stop and be punished, and the child to get help and support... and until very recently that was not even a remote possibility. So... maybe a better question is "why do people report abuse, and how do they do it in a way that actually benefits the person being abused?"