Because we don't KNOW that and we cannot know. I know it looks bad, and personally I believe it was an accidental leak from that lab. But that's just me: we have no evidence! I bet the Chinese know, one way or another.
I have trouble with believing it was biowarfare because China got it too and right now have some sort of dire situation working in a large city --- but mostly because I know that China has been the main source of pandemics, especially but not only viruses, for millennia, and they were never biowarfare before, except for the Black Death, of course. But that was really the Mongols who threw the corpses over the wall, not the Chinese. Mostly, pandemics just start in China and float out of there as far as they can get. China really needs to start getting ----------- cleaner. I don't know what their problem is.