Why do the majority of people fail to keep fat off?
Long term weight maintenance remains a very difficult task indeed. Healthy weight loss is clinically defined as losing only 5-10% body weight, but weight is gradually regained in the vast majority of people. Previous research has shown 95% of people regain lost weight and recent research has shown that ~20% of overweight individuals are successful at long-term weight loss when defined as losing up to 10% of initial body weight and maintaining the loss for one year (unfortunately not great).
This is because weight loss maintenance is hindered by complex interactions between environmental, biological, and cognitive factors controlling weight regain. Anyone who thinks that maintaining a healthy, fit looking body shape and weight is simply a matter of exercising more and eating less are delusional and ignorant to the complexities.