Because if game world is filled with beautiful people and all defects are removed, then the game world feels atrifical instead of being immersive. That's even more important for the main character, as this is the person you're supposed to identify with, and that's going to be shallow if that person lacks any substaintial flaws or reasons to be insecure. Furthermore, conventionally attractive people are (aside from being boring to look at) treated differently in society, which would either have to be reflected in the story and change the dynamic and struggles the character was supposed to face, or not be adressed at all and feel atrifical. Next we have representation, which is obviously the issue for the incels here. Ciri in the trailer is hot as fuck. The very idea that's she's "not enough" for any of them is laughable. The issue they have is that she's a woman, something they despise and see as inferior to themselves, being portrayed as a powerful human being with unyielding character, standing against the traditions in the trailer to boot. That's the exact same thing they've been doing to discredit feminists for the past hundred years, claiming all of them are ugly and thus unworthy of engaging in dialog with. That's their signature and default position to absolutely any non-subservient woman in media, and claiming it's actually about aesthetics is bullshit so transparent they can't see their own reflection in, which is most likely for the better.

Also you have hundreds of games that cater to your desires, go play those. If you don't like something, don't engage with it.

Because stories are not always about attractive people, and sometimes that makes it way more interesting.

Ever hear of character actors?