As long as weight loss is extremely easy (let's not kid ourselves, there are much harder things in the world)

If it's so easy, why are diets so infamously ineffective, why do existing social stigmas and norms fail to ensure slimness, why is obesity so heritable and genetically varies across countries ("Population genetic differentiation of height and body mass index across Europe"), why are people exercise resistant, and why do the randomized experiments show much less weight loss than necessary for health ("Intentional Weight Loss and All-Cause Mortality: A Meta-Analysis of Randomized Clinical Trials" being the last I read, estimating long-term losses at 12lbs and half what is necessary for Americans)?