[This](http://www.smithsonianmag.com/specialsections/40th-anniversary/Nine-Historical-Archives-That-Will-Spill-New-Secrets.html\) articles talks about various information which remains classified and what it might reveal. Some of these are in private hands and so perhaps the time period is understandable but in many cases they are government documents and the people involved are long dead (often their children as well) and or technologys obsolete. Some of the time period are for a hundred years.
What the point, now that the information is at best only of academic interest?
The ruling classes have found and make use of this type of shenanigan to protect themselves and any future progeny from insane mobs, like those that formed in France circa 1789, and the Czarist Russia of 1917.
Who cares to find out today that Alexander the Great was an insane killer, even condemning to death his own army… finally poisoned by his own generals.
To the question in your title, the point of the lengthy classification times is obviously so the information will only be of academic interest. To the closing question in your OP, the point now is academic interest and ordinary morbid curiosity.