Bureaucrats hoping to expand their control over the Executive branch.
Documents can be classified for a multitude of reasons and some don't have anything to do with national security.
All of Obama's, and Presidents before him, Presidential records have been turned over to NARA.

Biden's classified documents from his days as VP were under which president ?
Biden's classified documents from his Senate days were under which President ?
From a NARA press statement on Oct 11, 2022 ..
"Reports that indicate or imply that those Presidential records were in the possession of the former Presidents or their representatives, after they left office, or that the records were housed in substandard conditions, are false and misleading."
And Senate Intel and NARA are now arguing over which, if any, of the Biden/Trump/Pence documents that they will be allowed to see. 🙈