Would you call the indigenous people in some regions of e.g. Africa or South East Asia also "sluts"?
Why do you need so many layers of clothing? Full body cover clothing is usually only a thing in regions that need protection from dirt (due to having less water to clean with), the insolation (i.e. not to get burned/dehydrated by the sun) and/or the cold.
Elves aren't even human, maybe they don't have the concept of "clothing" in the first place and feel uncomfortable wearing it, hence she only covers what is culturally appropriate in human society.
There are many reasons as to why, it's just that Japanese authors again and again don't spend any time on world building that their fantasy doesn't make sense and comes over as weird/fan service.
They often suck so much that they have to include Japanese staple food into the stories because they couldn't imagine building a society that doesn't live by that, which shows their complete lack of imagination and/or cultural education (maybe that "world history" is just something for show and in fact they don't learn anything about actual cultural differences; we in Central Europe at least don't make the claim to learn about world history and just focus on what influenced our own history and culture, so we can claim to be uneducated here).
Well,at least for Japanese readers, it doesn't seem to be something that hinders successful sales, so I think it might actually have to do something as to how Japanese people perceive culture as a whole,not just the people writing these stories.
So far the only story I've read partially that does try some serious world building is "Isekai Ryouridou", where you learn about living conditions,working conditions, cultural properties (food,customs,traditions,interactions), societal structure and you have different clothing styles as well as skin colours.
It's the only story that ever tries to establish a real society I've come across out of Japan.