Because The Elder Scrolls series is not set in USA.
Just go to any country that fast food is not prevalent, and you'll seldom find people who's really really fat.
And also, in medieval times there are much fewer fat people than nowadays. Most commoners during that period can barely feed themselves.
And yes, looking it technically, supporting multiple vastly different skeleton seems to be too harsh for this decade old engine now. I guess adding some slightly different builds is the most Bethesda can do now.
Or you could just say they are lazy and the audience who love making a fat character so too small that they didn't bother to add a fat model into the game.
Fast food isn't the problem. There are plenty of skinny people over here but who are incredibly unhealthy with all the fast food(so its still a problem) THe problem here is the lack of exercise. In the USA, machines do everything for us, and we can live our lives just fine just by having an in home office. Or we just take a train to work. In Tamriel, there is only manual technology. All the vocations require exercise. Even Sinderion, the alchemist who lives in a basemant, gets exercise from his field trips and his trekking across Tamriel.
So no, no fat people plz.