Here's the thing that is usually overlooked: the amount of calories consumed. Fat people simply eat too much.

How many sugars do you put in your Pepsi? (youtu.be)

Fatties eat the most cucumbers of any group of people on Earth and it isn't close.

Salads and vegetables in general are not satiating either calorically or micronutritionally. You can't just tell a fatty to eat salad since it's essentially starvation. They have likely never been exposed to real foods their entire lives and have to be taught how their ancestors would have eaten. Something as simple as cooking eggs in lard is probably alien to the average fatty.

Choice.

They don't eat it