Yeah but I mean what do they actually do if you want to open a door and someone is standing behind it? Forcibly push them back? It's as much a design decision as a programming one, since that sort of thing matters in multiplayer sometimes. What if you push them into an enemy? Enemies can block movement. Do you push them both? If there's literally a conga line of players alternating between teams, do you push them all? What if the line extends all the way into another wall? So many things to consider and I bet this is why Valve decided to just use sliding doors exclusively, much like why they decided against having elevators.
EDIT: I mean, when you think about it, it's pretty much the same reason real-life automatic doors are always the sliding kind. You only have to be sure it opens, or stays open, if there's someone near it. Which, actually, kind of solves your problem from the other side: There is no "manually open door" button in TF2, and automatic hinged doors (and automatic doors in a house, period) aren't realistic, so there's your justification for not having them. If you want realistic hinged doors, just have them always open. Why not? That's how they are in Mann Manor.