If you truly believe there should be investigations, you have to accept when the results of the investigations

That doesn't logically follow. It's like insisting OJ wasn't guilty of murder, because the criminal case didn't stick. But he was guilty of "wrongful death" because the civil suit did stick. What kind of conclusion do we draw when the police fumble the bag and private investigators continue to turn up incriminating evidence?

And even then, you can both have an investigation (even one that turns up culprits) and still have a cover-up.

There's even a term for it: https://en.wikipedia.org/wiki/Limited_hangout

According to Victor Marchetti, a former special assistant to the Deputy Director of the Central Intelligence Agency (CIA), a limited hangout is "spy jargon for a favorite and frequently used gimmick of the clandestine professionals. When their veil of secrecy is shredded and they can no longer rely on a phony cover story to misinform the public, they resort to admitting—sometimes even volunteering—some of the truth while still managing to withhold the key and damaging facts in the case. The public, however, is usually so intrigued by the new information that it never thinks to pursue the matter further."[