Because evolution. For millions of years, our ancestors lived from hand to mouth, and when "extra energy" suddenly appeared, it was beneficial to store it in the form of fat and spend little by little while the next hungry time lasts.
Relative satiety, and even then not everywhere, became the norm, perhaps less than a century ago. The problem of obesity in developed countries arose even later.
Possibly because the person is struggling to prevent this unhealthy fat from finally being used up and leaving.
What do obese people usually eat? Do they follow therapeutic diets (medical tables) or do they eat right?
They continue to consume more calories than they spend and fat only accumulates.
And if a person seriously begins to lose weight and changes his diet, adds activity - the fat goes away!
Bottom line: fat does not evaporate because we do not allow it to evaporate.