There's certain secrets which can never be revealed.
Imagine if America somehow caused the death of Queen Elizabeth ( as far as I know there's no evidence of this just an example.) It would take centuries for it be ok to reveal this with out ruining American English relations. This is intended as just a thought experiment to see how bad a country would take it.
And if you think "Nah they don't have anything that bad." The way America has fucked over the world with it's KNOWN foreign policy... there's 0 chance that we don't have many many many skeletons in our closet. I guarantee we've assassinated people. Potentially Leaders (no matter what Carter said). I guarantee we've interfered with elections, or even fully rigged elections. We've probably handed over dangerous weapons to countries. We've almost certainly done devastating things to multiple people in the world.
And think of everything we've done to our own people. Just think of what we HAVE admitted. The Tuskegee Experiments? That's what we HAVE admitted to. I'm guarantee as well that's not the worst thing our government has done to their own citizens. I mean imagine if we find out the FBI threatened the president, or actually did kill Kennedy or even just any involvement in that? Holy shit, there's "Think something happened" And there's "Know something happened"... that's not getting revealed.
Should we have an automatic sunset for classified documents? Sure. Should we stop doing awful shit? Yes. But my god, the amount of skeletons in our closet and what will happen when it's all out in the open.... it won't be released. Hell I'm willing to bet there's thing that aren't even shared with the president (Maybe because he doesn't know what to ask for or they flat out wouldn't share it with him. There are limitations to what is shared with a president, for instance an investigation into him.)
Or maybe all classified documents will accidentally be destroyed in a fire the day before they're to be released. OOPS!