Because it matches the setting??? They're in a southern island of Japan. Asian people, including Japanese people, can vary skin tone extremely heavily based on sun exposure. There's also genetics at play as the southern islanders have a lot of Ryukyu blood in them which results in naturally darker skin tone even when not tanned. It's used to indicate whether someone is "country girl" or "city girl" as well in Japanese fiction.
Bitch i ain't complaining about that, It's just really rare to see dark skinned characters in Japanese media, and when it's done, it usually turns out to be just a tan girl, it's just too much of a common occurrence, that's what makes me lose my shit, not whether it makes sense story wise or whatever the fuck u sayin
