Probably because they don't consume gobs of HFCS and ultra-processed foods, don't take the car for every single thing[0], and have obesity/overweight rates that are 20-40% lower. A healthier work-life balance and concomitant lower cortisol and blood pressure also helps a lot.

If you compare Italians and Greeks to, say, Swedes and Dutchies, you'd get a much different picture.

[0] not entirely Americans their personal fault, their urban design isn't for walking around