Maybe they did attempt to remove it and the conductors were so short the device only pulled out a quarter inch max. Plan B was then followed.
How would that device be able to be installed in the first place?
Ran across some old armor cable, with the rubber covered wires, feeding an attic j-box yesterday. 90 years worth of heat and age made the insulation very brittle, and the original taped splice a tarry mess. Didnt really want to open it up for fear it would turn into a cluster, but I prevailed. Maybe the person who installed this outlet-over-outlet had the same thing once, and decided not to open up a hornets nest again. Not saying it's right by any means, but I can see one way this could have happened.
Kinda surprised looking at the pics again that the wall wasnt cut back and a cover plate put on the original receptacle, leaving a recessed design. They probably wanted it to look good at the expense of code and safety issues. Reminds me of the saying "Work it may, but shine it must".