I've always been confused by this too - you'd think holding your breath would make it safe-ish. I can only assume that it's still extremely risky because even

I've always been confused by this too - you'd think holding your breath would make it safe-ish. I can only assume that it's still extremely risky because even if you remember to do it, you are very likely to misjudge how long you can hold your breath during an emergency situation with adrenaline pumping that requires you to drag someone up the stairs. Which means you get halfway up, end up forced to take a big deep breath, and pass out.