I think this was discussed before. It was a continuity mistaike, if not a moment they just wanted to be "dramatic."
EDIT: Well, for one, the host needs to stay alive, and for it to stay alive, it would need to breathe. Second, the Chestburster would kind of gather DNA or something of the host, need be, staying alive. If not, it would be horribly disfigured (as if it wasn't in appearance already).
"To me, everyone is crazy. The ones in the mental house just got caught."