Well, Biden had some in his garage, didn't lie about having them and returned them when asked to, that's not what Trump did.
Donald Trump privately expressed concerns that turning over potentially classified documents in his possession after a May 2022 subpoena could result in criminal charges while repeatedly engaging in what prosecutors have described as an effort to enlist his lawyers to lie and destroy documents...
abc7ny.com
As much as it's a problem that they had these documents in unsecured locations, for Trump the crime was in lying about having them, and conspiring to keep them in spite of the request by the national archives to return them.