But a door (with squeaky's head between it and the frame) has no resistance (other than squeaky's head).
So now put squeaky behind a closed door and get three men to push against it if it has a lock and a couple of frame bolts - it won't move. But if they shoulder charge it it probably will.
And this is where baffles come in useful in the tank - they make a difference too. Just think of a tanker lorry with 25,000 litres of fuel in it - why doesn't the front end of the tank (bulkhead) come off when it brakes or hits something? Baffles.