A few loosely related points here:

-Losing weight takes discipline and dedication.

-$1 will get you more calories of “junk food” than “healthy food”. Soda is cheaper than water. Not everyone has the luxury of choice there.

-There’s a psychological component as well. You’re happier eating a cheeseburger than a salad.

-There are bigger seats, with much more room, and all the snacks you can eat. Usually located closer to the front of the plane.