Biden illegally removed documents, stored them for decades, then showed stuff to someone without a security clearance.

There's no evidence he illegally removed documents. Again, he almost certainly wasn't the person who actually packed or moved the documents - and it's very unlikely that he had any granular review over what specific pieces of paper were packed or left.

Nor is it a crime to store classified documents for decades. As with Trump, the crime occurs when you refuse to return documents that you had lawful access to at the time you got them on request. It's the refusal to return, not the retention, that's the problem.

Nor is there any evidence that Biden "showed stuff" to someone without a security clearance. He read passages from his private notes to his ghostwriter. Those documents would not have had classification markings and wouldn't have been reviewed or assigned a classification category, and so it becomes difficult to prove that Biden knew that the information was classified at the time he provided it. That's especially going to be hard to win in court because of the government's longstanding and frequently utilized process for doing classification reviews for books written by government officials - the government is very well aware that classified information that isn't marked might be inadvertently revealed and want to take steps to review to prevent their publication.

Had Biden refused to give the documents back, and instead tried to hide them, he would have almost certainly faced the same charges as Trump. Because he didn't do that, and instead did the right thing, there's no prosecution.