The same reason sexy actresses lose relevancy after 25. The same reason porn is majorly "barely legal". Societal preference for young, innocent, untouched girls rather than competent, experienced women. Look at the male characters and they're about the same, clean cut teens with maybe an exaggerated shoulder width and jawline.
And also because anime is a caricature of human form based on, presumably, a race different than yours.
While we're at it, as far as mainstream Japanese society is concerned, those super-cute women wearing schoolgirl uniforms and tiny backpacks are actually bucking the male gaze by being too immature, apparently. It's not a sex thing, it's anti sex. Can't control the international gawkers though
There's a big fucking difference between a 25 year old actress and a drawing of a girl with a 12 year old face and EE size tits in a sexual suggestive position.
It's not a sex thing, it's anti sex
Again, the EE size tits, wet T-shirt, and suggestive position disagrees.
People downvoting this don't want to face the RAMPANT sexualization of children in anime. To the point where it is hard to find a series without an overly sexualized child or child-like "adult".
Do series without them exist? Yes. And they're better for it.