Why do some people stay slim, and never gain weight?

I'm going to assume that what you are asking is about people who have to make an effort to control their weight, that it doesn't "come naturally" for them. Which would be most of us, except that most of us stopped being slim at some point.

I'm also going to assume that "why" is meant to be what's their motivation, rather than how they did it.

You purposely excluded people "who are in the business of being seen (models, acting).

Is vanity what you're getting at? It's easier when you can use health as a more of a motivator than vanity, but there's no denying that appearance is a huge factor. At any age. Personally I have gotten into situations where I could feel stress building and see that I was starting to gain weight, but I was willing to keep going to achieve some kind of goal. If appearance had been my top priority, I would have made different choices.

Is it strength of character, resilience, that kind of thing? If I were mentally tougher I could have gotten through certain situations without overeating. I'd put that in the category of "come naturally" though.

You didn't exclude athletes, for whom health is a business. I'm having a hard time thinking of a sport where you can make your living in your 40's and beyond though, so maybe you didn't need to exclude them. Personal trainers and nutritionists, or others whose income depends on guiding others in healthy lifestyles, would be an exception. Like actors, they have strong motivation to stay slim.

So, people who are unusually driven to stay slim for vanity or health reasons is my final answer.

Like @CattOfTheGarage, I have family members who are naturally thin and only eat when hungry and effortlessly have the ability to stop when full. My own son is like this. He even forgets to eat sometimes. He is very active and slim. I on the other hand, I think of nothing but food, have no concept of the feeling of full and often binge inexplicably. How is it possible that I raised this man and he does not have my abnormal relationship with food? I have grandparents, aunts, uncles, and cousins with his normal eating behavior. My life long question is what is wrong with me that causes me to behave the way I do? And is it possible to rewire my thinking and behavior or am I doomed to be this way?