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
Men have been using sexy bikini pic women in the bios long before lemmy. Look at any hobby forum like bodybuilding, car repair, guns, whatever.