You say there’s nothing damaging in there. If that’s the case then the Biden campaign has even less reason to respond.
There's probably nothing really damaging. But some things in it could still potentially be interpreted that way, to the point that if someone had completely fabricated the emails, I think it'd be worth coming out and saying "to be clear, these are blatantly made up, my son never sent this email/these emails" even just once. Even in just an off-hand comment in an interview.
For example, here's a screenshot of a few of the emails in question: https://i.imgur.com/XNQarwF.png
Is this a smoking gun of corruption of any sort? Obviously not. But could it raise some questions, before an important election? It could. If someone had completely fabricated this email, in my opinion almost any rational person would, at least once, somewhere, say or write that it was absolutely completely fabricated. Additionally, someone on this exact email thread corroborates that the email thread is all real, and claims that "H" is Hunter Biden and "the big guy" is Joe Biden, which seems plausible given the email context.
I think it definitely is a cover-up. Not because of some collusion between Biden and the media; I think it's all social forces and incentives.
Optically, the media doesn't want accusations of having helped Trump win if he wins.
At the object level, a large percentage of people in the media lean left and strongly dislike Trump and don't want to do anything that might help him win.
And, probably, some percentage may also be so biased that they really think there's no possible way the emails could be real.
In the last case it wouldn't be called a cover-up, but it's being awful at one's job. In all of the cases, it's not a cover-up in the sense of a nefarious conspiracy, but it's journalists not being journalists due to strong political bias.
I think if this were leaked emails about Trump, the right-leaning media would've done exactly the same thing and not reported on it, or only reported that "the left-wing media is spewing conspiracy theories again!", as the inverse of what's happening here. I think the lesson that's reinforced here for me is just that all media organizations of any kind once again can't be trusted when it comes to actually caring about truth and epistemology.