Because it wasn't the exact same thing.
There's your extreme right wing bias again.
Biden discovered the documents, called NARA and cooperated to hand them over. Trump denied having them and went out of his way to hide them.
In both cases, the Archives reported the discovery of classified documents to the Department of Justice.
When the Archives informed Trump's people early last year that it was going to hand the documents over to the intelligence community for a review of any potential damage to national security, Trump's team objected and asked for a temporary delay. The Archives granted Trump's first request but denied his second, court papers show.
The FBI later obtained information that Trump had more government documents and issued a subpoena for their return. In June, Trump's lawyers turned over a packet that included 38 classified documents, including 17 marked top secret. Trump's lawyers showed the agents the storage room where the documents had been held but refused to allow them to look into boxes that were still inside, according to court filings. They also gave investigators a declaration that a "diligent search was conducted" and that "all responsive documents had been turned over," government court filings show.
The FBI then learned Trump had not fully complied with the subpoena and still had more classified documents. They obtained a search warrant for his Mar-a-Lago property in August and found over 100 documents marked confidential, secret and top secret in the storage room and in Trump's office.
Trump later sought to slow the feds' investigation by asking for a special master to review the documents and demanded their return, claiming they had been declassified and were his personal property.
Another search for documents carried out later in the year at the behest of Trump’s attorneys turned up two more documents with classified markings at a Florida storage facility used by Trump near Mar-a-Lago, NBC reported last month.
In the Biden case, Sauber said that since the documents were found on Nov. 2, “the president’s personal attorneys have cooperated with the Archives and the Department of Justice in a process to ensure that any Obama-Biden Administration records are appropriately in the possession of the Archives.”
Bill Barr on the classified documents investigation:
"This is not a case of the DOJ conducting a witch hunt...This would have gone nowhere had the president just returned the documents, but he jerked them around for a year and a half...There is no excuse for what he did here."
I rest my case.