It's a combination of a few things:

There's a massive amount of junk food and ultra-processed food in grocery stores, even though (rough estimate) 50% of floorspace is "raw" food. (Fresh fruits and vegetables, meat, fish.)

Processed food tends to have more sugar (high fructose corn syrup) than other countries. The same brand in the US vs another country will have more sugar.

Cultural momentum: Everywhere you go there's unhealthy food.

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Speaking from personal experience, junk food is just plan addictive and satisfying. It's not like alcohol or other drugs where you can just abstain; you gotta eat and we all get hungry.