There was an investigation of that. Lost in part because, in the interests of transparency he let the Trump-era Hur run the investigation. In the report Hur dwells on Biden's photographic memory. at one point Hur tells Biden "you appear to have a photographic understanding and, and recall of the house.", but turns it into a political hack job, where he claims biden is old and senile. The report makes if very clear he was sharp and the charges weren't brought because he didn't try to keep the documents.

The case history is that if you notify the authorities and cooperate promptly you don't get charged.

Biden noticed the documents (in a locked cabinet), let NARA know that day, and they were retrieved the same day.

The prompted a consensual search of Biden's home, and the other documents were found and retrieved.

Trump has engaged in a huge campaign running for four years of lying, delaying, moving documents so that they would not be retrieved, and trying to cover up that deceit.

Hence the search warrant, and the charges.

Also the concern that he has been or is selling the secrets to Saudi Arabia, or others.