Adding a hinge there would be extra weight, but probably more importantly, the plug door is convertible to an emergency exit, so you want the hardware to be similar to the emergency exit door. For those, the door comes completely off and you throw it out of the plane when you use it. If the door hinged inward, it would take up a lot of space in the cabin during an evacuation and slow down people trying to get out.

Ultimately it's extremely difficult to make a plane where you can just not install a bunch of bolts and everything still keeps working fine. You have to have a high quality manufacturing operation to make a reliable plane.