Yeah i've noticed that too. Just like there are absolutely no obese/fat/overweight/etc. people anywhere. Everybody is in shape. I find this to be very odd.
And people don't have freckles, scars or moles either. I'm going to let you on a secret -- one of the most closely-guarded secrets of the lore elite: to have obese people in the game, you need to make obese people models. To have people with different skins (so that they can have freckles, scars, moles, etc.), you need to make these textures. And you need to code the game in a way which allows you to arbitrarily give models and textures to specific people, rather than to an entire race at once. And all that is work that takes up time, time which could be used instead to put more loot and monsters in the game. Guess
which choice the developers are going to do?
How come? In the middle ages there aren't as many fat people as there are now... I think.
Tamriel is one thing. Earth's Middle Ages are a completely different thing. There are some superficial resemblances to
all eras of the real world from the antiquity to nowadays in Tamriel. It's not supposed to be the past of our world, thankfully! It's a different world altogether. And aesthetical considerations rank much higher than historical reconstitution as far as concept design is concerned.
And there were fat people anyway, the difference between now and then is that then it was mostly the rich people who were fat, and they caught the gout; while now a lot of lower- and middle-class people are obese too.