And what if they can't? I'm overweight, not obese but I can do very limited exercise due to my spine. I can walk (but < 2 miles) or ride my bike (< 2 miles). It's not stamina or lack of willingness as I love to walk and double that for riding my bike, but my spinal issues. Any fuck-ups = herniated disc at worst or bad headache at best.
I lost weight (13% of my body weight) but I can't lose more, and I'm having a hell of a time keeping it from coming back, even though I changed how I eat (not back to bad eating habits - I figure that the eating changes must be permanent.)
So it's not all lack of willpower or effort, sometimes your middle-aged body just screws you.