Of course there are multiple valid reasons to have doors as they are right now. To add to the existing answers (and excuse my english, I'm not a native speaker):
Having hinges (the metal thing that holds the door, had that translated) that open in both directions makes it difficult to actually really close the door. There always has to be some amount between the door and it's frame. Otherwise the door would be stuck.
This, on one hand, is part of the securitysafety-issue. Few doors are really air-tight. But having a visible gap would allow a fire within room A to get some oxygen from room B.
At least here in Germany, doorframes have some sort of rubber part that seals the gap between door and frame. This not only prevents air to flow, but also reduces the noise passing, since noise needs (although not exclusively) air to travel from A to B. Such a rubbar-seal wouldn't be trivial to incorporate into a door that opens both directions.
And a guess on top of that: those hinges that open in both directions might cost quite a few buck more than the regular ones.
