Because it could happen. It’s unlikely yes, but all it takes is one lucky mutation and we’re done. They were correct in that game, our understanding of fungal infections in humans and our ability to treat it is almost non existent.
We were able to product a vaccine for COVID, a far, far less disruptive illness, within two years (via huge global effort) because we’d been focusing on that area of research for decades very closely and producing similar treatments for a long time already.
But something fungal, and highly contagious? There’s nothing we could do except try to quarantine, bomb and napalm every infected area, and hope we got it all.
And we’ve already seen how an easy to contain illness like COVID simply can’t be contained even when we’ve had a heads up and some time to prepare. It will suddenly explode into the population, and once it’s out there it’s out there.
Long ago, we used to be protected from extinction due to disease as a species due to our inability to travel long distances to spread it. Now? All it takes is one infected person to spend a few hours at a large airport, and within 48 hours it’s reached the doorstep of vast majority of the populated world, and is already behind our best pandemic defences.
If a fungal infection that serious ever does make the leap to humans (which again while unlikely, is also entirely possible, it’s like winning the lottery - it could happen tomorrow or maybe never), we have an extremely tiny, almost non existent window in which we must identify how dangerous it is, quarantine the entire region it was located in, bomb it off the face of the earth and hope to the gods we got it all.
But, our morals, humanity and our indecision will stop us from committing what would normally amount to serious war crimes to save the human race, and that tiny window will slip by.