[deeviant]

Because you eventually have to eat. And also eating and hunger are heavily linked to all sort of hormonal/psychological impulses(ask anybody that has tried and failed to lose weight), staving yourself out for a few days is a great way to encourage a binge, which creates a massive blood sugar spike and leaves said diabetic far behind where they began.

Also, in pre-industrial era(well, pre artificial insulin), diabetes was a death sentence, although the treatment of the time was related to diet(high protein very low carb). However, just because something is a treatment, or relieves some aspects of a disease, does not make it a cure. The diabetes patients lived longer with the modified diets, but they still died in fairly short order and suffered through absolutely horrible symptoms.