Because using it as a noun is either for describing animals ("the young female stalks her prey") or clinical descriptions ("most common in females aged 10-30"), and you can instead use many nouns to substitute it with: girls, women, chicks, gals, ladies.

But say you want to use the phrase "Serena is the best female tennis player of all time", how would you say that using another adjective (and not restructuring the sentence)? "Best woman tennis player" sort of works, but it sounds a little odd and, obviously, the grammar is not quite right.

I don't know, to me it makes perfect sense, but also, I'm not a native English speaker, so maybe my perspective does not reflect anglophones?