Most people do not set out to become overweight, it is usually the result of multiple compounding factors. Metabolism slows with age, so people burn fewer calories with the same level of activity. So someone can start gaining weight without having changed her behaviors in any way.She is moving the same and eating the same, so in her mind, her weight should stay the same. But her body is using energy differently, and she is slowly gaining weight.
And the signs of weight gain can be mistaken for other things. A pair of pants may suddenly feel tight, but maybe they shrunk in the wash. She may suddenly feel winded climbing a flight of stairs, but maybe she was just feeling run down that day. She may notice she has a double-chin, but maybe her skin is losing its elasticity with age.
The point is that most people experience these signs independently, not interdependently, so they do not realize they have the same cause until they gain enough weight for the weight itself to be entirely obvious.
But at that point, the weight gain is snowballing. Once the momentum is going in a particular direction, it takes a lot of effort to stop it, let alone turn it in the other direction.
And our bodies like to store fat, it is a survival mechanism against famine and starvation. So it is much easier to put it on than it is to take it off. Especially, since there is a significant delay between when you are full, and when your brain registers that you are full. So it is easy for people to not know they have overeaten until 20 minutes after they have finished.
Losing weight, particularly large amounts of weight, is a lot more difficult than "wash your hands," "wear a mask," "get a shot," etc. That is why it is not useful suggestion for managing COVID. It does not offer protection now, when it is needed, but years from now, when it is accomplished.