Because POTUS wins the election regardless. If you knock out Biden or force resignation too soon, he just gets replaced with someone who could genuinely rally support and possibly defeat POTUS. They are trying to thread the needle - cripple Biden, but keep him in the race. Biden forced resignation equals calls for delayed election, delegitimize election, coup, etc. This type of leak allows POTUS to have free reign after the election and minimizes legitimate arguments against his victory..