1) People who don't manage their nutrition and maintain a healthy weight
2) People who manage their nutrition and maintain a healthy weight
3) People who don't manage their nutrition and have uncontrollable weight gain
Yes, if everyone who is (3) started going to the gym and counting calories as (2) do, then they would have an easier time maintaining a healthy weight. But most people can't do that. I can't do that for more than like a week at a time, but it doesn't matter because I'm a (1).
Why are (1) and (3) different? This has nothing to do with self-control or willpower. There's something off in (3)'s biological systems that calibrate their intake of food. The slightest caloric surplus leads to substantial weight gain over time. This question could occupy an army of scientists for decades, but the stigma against fat people has done serious damage to this very important medical research.