Because they're practical, almost sensible in a way. You won't get stressed out about other people's suffering if you don't care, I guess. As for how you can actually be that caring, I'd suppose it's something to do with genuinely feeling and believing that other people's suffering and opinions don't affect them. Just thinking out loud here.
Maybe they feel like they have too many problems of their own.
i agree..compassion is becoming a very rare trait
Because the world is cruel and we have to do everything we can to survive.