True, but there is a chance of humans being in the room when a fire breaks out. That's why they breathing apparatus scattered around (hopefully).
The last halon datacentre I was in had breathing apparatus within easy reach no matter where you were standing in the place. Two paces, at most, to grab a mask. You do have seconds if it goes off, and that mask is only to give you enough time to run out of the room.
The newer stuff is less hostile to organic life, so there's no reason for a respirator.