There's a good reason you cant easily 'mate' the back of the cab to a solid mount box on the frame: The cab is on hinges up front and airbags and shocks at the back. As you hit bumps the cab and sleeper is 'floating' on airbags. Not only does it move up and down about 4-6 inches or so, but there is also side-to-side tilt, like a rocking motion.
It is possible to join a frame mount box, but it involves the use of a special air-filled bladder or thick foam cushion all the way around the opening, which allows the cab to keep its normal movement even while there is a pass thru to the solid mount box.
Then of course there are structural elements in the back wall of the cab, but they can be moved and/or reinforced.
I suspect the builder wanted to be able to easily back up to a campsite and use the camper as intended without having to pass thru the driver and passenger doors, and up and over the air seats every time they enter or exit the living space.
Just my thoughts...it is an interesting use of whatever they already had or could get ahold of and make it all work together.