The most possibility is not because the first choice is a mainstream of conduct, but rather because of programing reasons.

You see, every outfit in the game need to be fit with the character. If the character is too fat or thin, the character will be clipping through the outfit. If you need to work on each outfit to make sure it fit perfectly on each type of body, that would be a nightmare since there is not just 3 type of body, but a whole slide of body type.

Sure there is the way to make your character skin disappear when your character worn an outfit, but that only work when your character worn a suit that covered an entire body. If the clothes is a little bit skippy, that's a lot of work for ya.

So, I think only a handful of game would sacrifice something so trival to make sure the outfit would fit with perfect realistic body, so they took a shortcut. And that's it.

tl;dr : make perfect realistic body types in game is not really worth it for how it can be made. So many game developers rather listen to people whining about unrealistic than add unnecessary works for them. Drawing a perfect realistic body might be hard, but make it work on game is much harder.

Some games simply has barrier in the story or the gameplay that the character appearance can't cross.

And then there's Saint's Row