Why are thick eyebrows so rare in anime/manga?
because good thick eyebrows are rare in nature as well, while it's easier to artificially have good looking thin eyebrows
thick eyebrows are cute and expressive, I say, and provide the ideal contrast for fairer haired girls to direct attention towards the eyes
Because most of the eyebrow power has long since been drained
thicker eyebrows means more masculinity. Thicker eyebrows in girls means that they have more testosterone, for instance they are more aggressive.
If you like thicker eyebrows, you like aggressive women.
I'm afraid the chosen one, Eyebrows, drained an enormous amount of it not too long ago and so there's barely any left.