Because publicly stating that it may be man made or originated in China would have achieved what exactly? Perhaps being a grown up means you have to play politically safe and base it on the actual provable evidence at the time, rather than gut feeling. Perhaps because he could predict what trump and his brickshow of an administration would have done if he didn't play a straight hand in public and how many more may have died rather than just the 1 million that Trump and Biden managed to bungle their way to.
His private thoughts are his private thoughts and not what is best practice in reality, and in fact all policy decisions were made by the administration rather than him.
I can totally understand taking the 5th to prevent that conflict between his personal thoughts and his professional conduct being decontextualised by Paul, Trump, the Brain Worm or Dr Oz, be it for a soundbite gotcha, a further criminal proceeding or just as political BS.