You can see why Trump and now probably Biden can get away with not releasing the last of the withheld files. 95% of the population have no idea there are even files being withheld nor care that they're being withheld.
95% of the population have no idea that the U. S. government, all governments, keep secrets. You cannot infer anything from a government keeping secrets.
For over 75 years the U. S. government kept secrets about the Civil War. But when the secrets were finally too old to cause anyone any harm, they were released. They contained information the KKK would have liked to have had. Names of people who spied on Confederate troop movement for the U. S. government. But nothing about a government conspiracy to assassination President Lincoln. And yet, millions of Americans assumed there likely was a government conspiracy to kill President Lincoln. Why else would they withhold those secrets?