Many Asian cultures see calling someone fat a compliment. It means you have more than enough.

Of course there level of morbid obesity is also much less.

Because it's generally not nice to point out people's shortcomings and struggles. It's not that hard of a concept to understand. I wouldn't call somebody ugly or dumb either.

It is called common courtesy. One says chubby or something softer like that.

I find saying "do not eat that or you are gonna get fat" ok, but "there, next to the fat lady" not ok.