It isn't intuitive how aircraft doors work, but the door doesn't open "out" in the typical sense. It actually sits against the aircraft's outer hull, the larger the differential the higher pressure exist between the door and the skin of the aircraft.

When the door is opened, it actually slides in then out sideways rather than swings open.

See: https://en.wikipedia.org/wiki/Plug_door

Most doors on aircraft open inward, so think of it like a drain plug where the pressure holds it in place.

Edit: To clarify, they may swing outward, but they open inward first.