Mostly, we don’t get enough exercise.
i've seen the data on this and a lot of people would be shocked
the USA has more gyms, yoga studios, sports complexes, school sports then every other country combined and more. we are actually a very very exercise obsessed country
go to any city in america and there seems to be a gym on every corner
you are expected to play a sport, just ask anyone no matter their age what sport you play or what sport did you play. people will say football, baseball, basketball, soccer, hockey, track, etc. i see a lot of fat people at the gym on treadmills and doing cardio every single day who stay fat.
but we also eat more than any other country and consume EVERYTHING more than every other country. food portions in america are like 4x more than other countries.
let's take the blue zones for example. do they really exercise more than us? they garden, they walk around town. but they don't do rigorous exercise ever like we do in america. they don't play organized sports either. they don't burn the calories we do from exercise that is for sure.
the trick is they eat way way less than we do though and their diets are completely different. we are talking 2 meals a day at most and tiny portions. meanwhile in america its huge breakfast, brunch, huge lunch, snack, snack, dinner, snack and snack. and if you don't get 300g of protein a day you are a wimp vegan who should get flamed. that is the difference