Genes are a gigantic part of it. I know people who cannot lost weight while being active and at less than 800kcal per day, and others who can sit all day long and eat ~3000-5000kcal and not gain anything.

Overeating can cause weight gain, but there's a lot of people who overeat without gaining weight, and others who eat little and still are overweight.

People are being brainwashed with crap like "calories in, calories out" or "exercise more" but that's just bullshit. It pushes the idea that too much weight comes from a personality flaw and not health issues, but yet any actual (and by that, I mean proper) doctor will quickly search for medical reasons for weight gain because there's a million of them. And a billion that are not known.

Telling people to have healthy habits is good. Telling overweight people, specifically, that they need to improve their habits is discriminatory and serves no purpose other than to dismiss health issues that push weight gain.

Fatphobia stems from a centuries-old view of overweight people as the "evil rich lazy politicians". Now, the rich ones are the most skinny, muscular ones, because it's actually hard to be reliably "healthy" (I do not count those as actually healthy, but well) without a shitton of money. As a result, people with weight issues are generally not rich and whatnot. We're not in the middle ages anymore and maybe it would be time to adapt the views on weight to the current reality of science and society.