Because it's not the only factor. Culture is an important factor but not the only one.

Income is another factor. Richer people had access to better food and better medical advice that pressures then to eat healthier.

Genetics are also another factor. Some people are better at accumulating fat than others.

That other elements does not remove cultural influence. You eat what is available to you in your region, what your parents cook, etc.

e.g. A country may tax sugary drinks to reduce availability and that reduces obesity in the country.