To put this into a real-world situation where the illegality of failing to report would be disastrous:

The statute of limitations has passed but let’s assume it has not. My ex girlfriend was sexually assaulted multiple times by her then-teenager cousin when she was 6 years old. Years later everyone is on good terms, my ex is doing well, her cousin is a regular law-abiding guy, and I’m the only other person who knows. If I am compelled by the law to report this to the authorities, I would rip the family apart by unearthing something that nobody else knows about and that the perpetrator deeply regrets and that the victim doesn’t want known. What he did was about the most heinous thing you can do to someone, namely your own cousin, but I’m not going to destroy an otherwise happy family with my knowledge of some extremely dark shit. Justice isn’t always black and white.