You hear about the illegal things federal agencies do almost entirely because there is a standard automatic declassification date that is about 25 years after report creation for most agencies. See Executive Order 13526 from Obama for more info as to why. Similar orders existed prior to Obama, but generally for longer time-frames than 25 years.

But understand that the actual spooky stuff gets reviewed at the end of those 25 years, and does not get declassified. It gets determined to be a "continuing threat to the safety of the United States", and remains classified. This is the vehicle that a majority of JFK and MLK's assassination files have been recurringly re-upped for over 50 years (up until Trump's recent EO demanding the files get released [which is an absurdly terrible way to get these files declassified IMO but I guess we'll see how it goes]). All it takes to justify making a report "not ready for declassification" is the agency director's say-so, which can be for any reason including that it would foster a bad view on the federal government and/or it's agencies from its citizens.

Anyways, this usually leads to FOIA requests on intelligence agencies (some agencies more than others) being nearly useless, because anything spicy will not be labeled unclassified and released to you. I think it's still a good idea to submit FOIA's to them anyway because it's always fun to make the FOIA offices squirm with their 100th annual request for information on Bigfoot, and it's not illegal (yet!) to do so.