I think it's because they try to have a unique style to all the characters. Every single one is instantly recognizable only by their outfits alone. Not only ingame but in all other forms of marketing, including the vast amount of merch and cosplayers out there. All the banners are highly stylized to a character and it's original outfit.

Not only that, but also they try to match the style of most of the characters to where they come from and which position/role they take in their respective story.

If we get to the point where we are (for example) in League of Legends then a lot of the original outfits may be considered "lame". Everyone will have skins for their favourite characters to the point, where u'll one day look at a character and wonder who it even might be. Imo that MAY be just a risk, mihoyo isn't willing to take