Part of it is because a whole lot of professionals who work behind the scenes aren't interested in these political games, they want to do their jobs, and they don't want to sink to the level of trying to score political points.
Often it's those people who are making these decisions.
Biden may not have been personally interested in directing them to go political like that. He was focused on other issues.
But mainly I suspect the files don't really exist in the first place, at least as described. What we've already seen might be the long and short of it.