That is the sort of belief we're talking about, a serious investigation with the assumption the person reporting it is not lying. The investigation itself could of course result in not sufficient evidence being found. It's really not that complicated.
Alternatively, look at the public scrutiny of police shootings. However you personally feel about them, there is a section of the population that sees them as a miscarriage of justice that is playing out in the public square. People look at the extremely low (non-existent?) conviction rate of officers and see either a police force operating without consequences or a remarkably well-mantained and well-monitored police force.
Obviously there should be an investigation, but history tells us that it is very easy to fool ourselves into believing we've done a "fair" investigation which will not look fair to outsiders or victims. No malice is necessary, but it is frequently endemic anyway.
That has the nice property that it's illegal, the whistleblower can get compensated, but only if they already have the means to wage a legal battle.
Investigations don't help much in these cases. The pressure isn't to find (often non-existent) evidence, it is to commit to the shitty process of trying to get people to care about you a painful part of your life.