At the end of "No Exit" why doesn't anyone leave when the door opens?

True to the idea that "hell is other people," all three characters are bound to an infinite, inescapable fear of one another. The open door shows just how drastic this mutual bondage is. Garcin, for example, is so obsessed with proving to Inez he is not a coward that he disregards what seems to be an obvious path to freedom. The point is something like this: the true "walls" that imprison the characters are not physical barriers--they are instead the psychological horror that comes with being forever alienated from one another. Even if one or more characters were to "escape" the hotel room, there would be no escaping this far more terrible social alienation.