Probably because it reuses parts of the design of the emergency door so it can be swung out of the way for maintenance. The emergency door uses this complex designs of stop pads and guide rails etc to have “plug” characteristics (which did hold out for 3 months without being properly secured) because a true plug door would have to swing inwards, which isn’t an ideal property for an emergency escape, and I’d guess it might need more room reserved internally.

Good point. Boeing (737) has the door open to the outside. The Airbus (320) the door opens to the inside.

I can remember when I was small my Mom mentioned that doors of buildings in the US are mandatory to open to the outside, a rule that does not exist in Europe AFAIK. So there you have it, you have a significant higher chance of crashing in a Boeing, but when it happens, your can leave the plane 2 seconds faster.