Naturally you put healthier stuff inside compared to some cheap quick to make fast food stuff. And if +- whole nation is having this sort of mindset, you see result when rest of the world is moving to cheaper-and-faster. Also, french are comparably fanatics to being active/outdoorsy (while having properly beautiful country to utilize). Good mix for good health.
Now why China and India see similar trend can be a different topic - their cuisine is good to excellent, but mentality, genetics etc is very different.