-Bloo-,
Yautja,
17 years ago
Chestbursters don't simply fly out. Facehuggers are to make sure that the Chestburster doesn't hit anything as it exits out of its host. They aren't that stupid.
They don't fully exit the host (at first), either. As you can see in any of the Alien movies (aside from Alien 3), maybe half of it emerges, you never see the whole thing.
They also never exit too fast. If the host is standing, they'd probably be laying on the floor or leaning back or something by the time the Chestburster peaks out.