they follow the golden ratio of beauty
the idea is for supermodel-like beauty, and if the subject's features deviate from such ratio, it is a challenge to use sufficient effects in order to compensate for the fact that they look less perfect-looking. Otherwise it could be seen as an artistic error that the eyes are too high up, the jaw is too wide, the lips are too thin, etc. Basically, pretty people tend to look the same
It's just because it's somehow become a rule that every female in the fantasy genre has to look like a Victoria's Secret model, I don't really like that fact either. It rather calls for a lack of induviduality.
Also, keep in mind it's for the cover of a magazine, so it's a bit more commercially orientated than Charlie's other images, which are all very unique and unconventionally beautiful.