Obesity itself is probably too vague and non specific to be usefully called a disease. The basic measurement for obesity, BMI, would classify bodybuilders or strengthen based athletes as obese. Bodybuilders and strong people can often be unhealthy, but it’s not quite the same as someone with high body fat.

Even then, people who are obese because of high body fat might have their lives shortened through multiple mechanisms. It could be heart disease that kills them, skeletal problems due to weight, immune issues, digestive issues, practically every organ can be affected. Any or all of these things could occur in a chronically obese person, so even if pathologized, obesity is less useful as a diagnosis.

Excess weight is bad, but you can calculate weight/height without a doctor. Focusing on better nutrition, eating habits, and exercise is the solution, even if you’re young and have a high metabolism. That sugar in our food needs to be taxed or regulated, as economic incentives drive obesity rather than people being uniquely stupid or culturally degenerate.