Reporting up the chain of command in a structure that supports and encourages such reports is one thing. We are talking about an unsanctioned action that you take against the express or implied desires of those above you. And a legal system that will back them up, despite you having definitely done the right thing.

The downside risk to yourself doesn't have to be existential for the upside to not be worth it.

Coming at this from a completely different angle, other people may see their duty to the law and the agreements they've signed as being more morally binding over their behaviors than their social obligation to raise the alarm. Not everyone uses the same formula to calculate which duty is most important.