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.
It's also done this way so a real emergency exit can be installed instead of the plug if the airline (or another airline that buys the plane) decides to increase seating capacity. It would be really inconvenient (not to mention mechanically questionable) if you would have to weld additional stuff onto the fuselage to accomplish this, so the parts that are connected to the door frame are already there...
