talk themselves into believing it doesn't matter for <stupid reason>

That stupid reason is, at core, nihilistic solipsism - and it's not as stupid as you'd think. I'm not saying it's right, but it does happen to be the one inescapable meme-trap of philosophy.

To quote your own fic, their reason is "why not?" - and their consciousness was not grown such that your impassioned defense of compassion and consideration have any intrinsic factor in their utility function.