To avoid ridicule. We mistreat people who struggle with obesity. We also skew the terms as to what a healthy body looks like. Weight is by far the worst gauge to measure physical healthiness. Body/height proportion, measurements, body fat %, lean muscle %, etc., blood pressure, cholesterol are far more effective.
As a personal trainer, the hardest thing for clients to come to terms with is their body image, that they must somehow be a certain weight or shape. To avoid the pressure and stigma, a lot of folks will say they only eat 1200 calories a day, work all day, workout, etc., and omit the parts where they don't record their calories. Why? Because we treat someone who struggles with weight abusively and unfairly.
When our cultural perception of a healthy body adjusts to being healthy relative to OUR body's natural way of being, excuses will diminish. Healthy relative to our natural way of being might merit asking ourselves: do we have hips and are busty? Are we naturally very thin? Are we bulky and muscular? Or lean and gaunt?
Voluptuous and curvy does not mean obese. But keep in mind, it's not a euphemism for obese, either.
When weight stops being a measurement we think is the end-all-be-all for healthy body image, it will become a lot easier to become healthy and physically fit.