It's a good question. Anime (like many media, but especially anime) is known to have gratuitous fan service where girls/women of all ages are in revealing clothing for seemingly no reason except to just entice viewers.

The reasoning is that because they aren't real people, it's okay to draw and view images of anime, regardless of their age. And because geek/nerd circles tend not to socialize with real women, we get this over-proliferation of anime girls.