I'm not knocking my door, but

 1/16 plywood sheet, honeycombed paper interior, another sheet.
*How* do you replace that?

Over the years, former people have installed chain locks into the really flimsy door, which must've fallen off, so they tried it again.
And again.

When stuff is cheap, you're careful with it, I get that. Part of my rant on faucets was about that.
(when stuff is expensive it could still be flimsy, but that's a rant for reddit or amazon)

reinvention time (very labor intensive)

The door should hang on the same hinges and other than a screwdriver and a fancy sander....
Um... 

That convenient-looking picture above, you'd have to destroy thresholds and the existing frame, then plaster the hell out of the cracks and spaces around the new frame.
No biggie for a $$$professional$$$ door-type-person, but still.
On the other hand, my threshold is really bad (really bad), um, 

My raised foundation doesn't actually NEED a threshold, just a larger door, hmm

Maybe by some ^%$# miracle and more hot days, 

the frame will cease to be swollen, 

and with some scotch tape, the door could still work.
It's more of a privacy-barrier anyway.