There is a vast difference between eating shitty food once a day while being able to walk everywhere and eating shitty food three meals a day and not walking anywhere.

The US both massively overeats the shitty food and is very sedentary for the most part. A bit contributor is our absolutely terrible work culture that wears people out so much that they seek pleasure from food and entertainment in the few spare hours they have each week because they are constantly advertised to encouraging that behavior.