If I look outside I see a lot of people who are fat who constantly go to McDonalds, drink Coca Cola and eat bags of chips.
Sure genetics have some influence and skeletons are different to everybody. But the big and only significant factor is the energy balance.
I know so many people who are actively trying to gain weight and are simply not able to. These people go out of their way to take in as many calories as possible and yet it doesn't happen for them. In your mind, everybody's body works and responds the same way.
I would caution you to stop talking about subjects you clearly are ignorant of.