Well, as to why someone would allow their health to reach such a level. I think that it's really complicated.
Something similar happened to my exbf's father. He used to patrol one of the highways here, back when they patrolled, so the story went. He'd stepped on a tack, and hadn't realized there was an infection. There's always been a sense of "I'm tough, I can treat it myself." In society. My grandparents grew up in an era where one didn't just didn't buy health insurance and doctors housecalls were pricey. Not that in today's day and age people don't self diagnose all over the internet.
So back to the exbf's father, whom I had grown quite close to, I'd call him Dad, he'd call me Daughter.
Yes, there was diabetes, but which came first, the infection or diabetes? I'd seen the gangrene. Seen the amputation (s). Visited the rehab as he learned to walk on prosthetic leg(s). I remember the dialysis and the eventual kidney transplant. I saw how easy it is, once one's health reaches that point, it's an uphill battle. Not that he didn't have positive energy. I remember being the last person he spoke with on the phone before coma. And saying soothing words to his fear.
It's easy, so to speak, to battle depression with health issues.
Sores happen from not moving and bathing. Sounds like depression.