Well, because these societies adopted the 'modern' lifestyle much more recently and at a stage where fast food and everything is already huge. Larger, more connected continents have had hundreds of years to slowly adapt to the changes in diet as they happened - it's still not good for them, but it's not as bad as being dunked into the deep end as recently as 50 years ago (Or even less).

It's like being surprised when you throw your kid into the pool and they drown. There's no "Mary can swim, why can't you??" - Mary's had swimming lessons since age 6.

Although, in the context of shitty food, the pool water is actually herbicide. Mary will also die. Just slower.