collapse of the old society

It's a thing in a good number of humans. Once you do pretty much any strength training (or just heavy lifting) at all, BMI loses its value pretty quickly. It's a rough indicator.

The DSM is definitely not intended to be stand alone. It is a set of general guidelines and definitions to inform the evaluation of a mental health professional. Most people check a good number of boxes on a good number of those checklists. The checklists are a tool to be used in collaboration with the professional judgement of the doctor. Almost every individual checkbox is "checked" or not based on the doctor's subjective evaluation.

Muscle mass and general body composition are part of it. Some people are naturally bigger and healthier at more weight than others. Some are naturally smaller and healthier at levels that would be unhealthy skinny for others. Presence of other obesity related illnesses are another. If you're showing signs of heart disease, breathing issues, etc, in addition to being big, that's a sign that losing weight will improve your health outcomes. BMI is a very rough yardstick that is used for the purpose of evaluating populations over time. It is not a good way to look at the health of a single individual without context.