It has nothing to do with spirituality or morals. Skinny-country person moves to the US, they get fatter (statistically speaking). They weren't skinny at home because they were better spiritual warriors or whatever, but because they didn't live in the US.

The options are to fix what are probably a whole bunch of problems across multiple domains at a cost of $(enormous sum) with a project spanning many decades (and which may easily be derailed and set back years and years at any time), so that living in the US doesn't make people gain weight, or... drugs, that work today. From a policy perspective, those are the only options. There's no good reason to think that reversing "moral decline" or whatever will help, since that doesn't seem to be why some other countries are skinnier.