In the film, Mia can still touch things and requires people to open doors for her if she wants to move from one room to another. Considering that she's having an Out Of Body Experience at the time (meaning her current form has no physical presence), her hands shouldn't be able to make contact with anything and doors should not represent a barrier to her.
This may be Our Ghosts Are Different. Even though she can pass through things, she still may see herself as solid, and since her living body is blocked by such doors, she can't pass through them but neither is she alive enough to open them herself. This is actually Fridge Brilliance, as her state nicely depicts her indecision of whether to stay alive or move on.
