Because classified records don't always come with expiration dates on their classification status for obvious reasons - even if records are classified for a speficic reason you'd probably want to make sure that reason doesn't still apply ten, twenty or fifty years later. You seem to assume the default is for information not to be classified. The right question to ask is: what incentive would there have been to declassify them earlier?
As others have said, declassification is a process, not a rubber stamp. Declassified records can reference things which are still classified so you need to go through each document line by line and check for such references to make sure they're blanked. Likewise if you want to be particularly helpful you'd have to also go through all previously declassified documents referencing this document and then un-blank their references and republish them, though I doubt that often happens in practice.
As others have said, declassification is a process, not a rubber stamp. Declassified records can reference things which are still classified so you need to go through each document line by line and check for such references to make sure they're blanked. Likewise if you want to be particularly helpful you'd have to also go through all previously declassified documents referencing this document and then un-blank their references and republish them, though I doubt that often happens in practice.