In the US the people elect a government, the government passes laws creating rules (or passes laws delegating rule-making authority which is functionally the same thing) for, among other things, how information gets declassified. That information then gets declassified or not in accordance with those rules.
> we don't know what we're missing.
That is literally always true, it's impossible to prove that the government isn't hiding something.
Considering that voting rates are around 60%, eligibility rules are often a joke, and most of those votes are typically wiped out by FPP anyway, it's more like some people elect a government...
This said, the US declassification system is as good as it gets. Most countries on the planet don't have anything comparable, and anything deemed a state secret will forever remain as such in those countries (at least officially, bar revolutions). Not even Western Europe is particularly good in that regard. Somewhat ironically, and particularly on Cold War matters, us Europeans are often keeping secrets that would otherwise embarrass the US government, which wouldn't be possible if US agencies kept them on their books.
> That's the idea, and by and large the system functions as intended.
What makes you say it largely functions as intended? It sometimes functions, but that is different. Also, we don't know what we're missing.