I don't think it's that weird for it to be a situation where they have to fix the problem to make the door open, denying them the easy option of just taking the problem out through the door. It would've been much worse if they just didn't think of it.
But the story didn't specifically say they couldn't fix the problem that way. So it just seems like a possibility that never occurred to the writers or the characters. It never occurred to me for decades, but then one day I realized, "Hey, why couldn't they just have taken Edith back with them?" Or at least, why didn't they even consider it as a possibility, even if they then ruled it out? Whether we can think up explanations decades after the fact is irrelevant, because the point is that the episode itself neglected the possibility.

