Because all of the major players, the Chinese Communist Party, the US government (DARPA & the NIH), and several universities and their IRB boards (UNC-Chapel Hill, WIV, and others), all had a hand in this, and the second they give legitimacy to this theory--that the hubris and recklessness of scientists and bureaucrats led to the deaths and suffering of millions--then they'll never be able to put the cat back in the bag.

They also know that if it was their fault, the CCP did a good enough job of scrubbing the evidence that we'll never meet the standard of evidence that would satisfy many of those who choose to take the authority at their word. We're seeing this first-hand in this thread.