When I posted Why Should They Care? last week, I was trying to capture how recent and rare the whole "brotherhood of man" approach is, and how humans outside a narrow grouping actually seemed to our ancestors. It occurs to me that we can only approach their thought if we imagine sentient non-human species. Even those may seem too close, such as has been the influence of Star Wars and other interplanetary fiction, suggesting that we might indeed be pals with non-humans.
But for a moment, imagine there are only a few hundred of us humans left, and all of them of your tribe, known to you, and of similar values. Their life is ever-precarious and the other sentient or semi-sentient species you encounter are not nice, or noble, but mere competitors for resources. Trading partners at most, wary allies only when under pressure from some worse oppressor.
You would, quite frankly, not care much what happened to the dog-men or those thieving fishy creatures who often prevented your tribe's access to water. If they experienced pain or want, that would be their problem and no concern of yours. If you needed to use them or take advantage of them you would do so, much as we saddle a horse or cage a chicken now.
An this would be an entirely rational approach on your part. The gradual expansion of who is considered important enough to have rights is the aberration. When we wonder how people could mistreat servants, or even own slaves, or torture other humans for amusement, we assert what an amazing and blessed time we live in, that such ideas barely have a foothold in our thinking. Not only time, but place, as most countries of the world are more like our ancestors than like us in this. They represent far more the lot of mankind than our present arrangement.
Actors like to trouble directors by complaining about a bit of stage business "What's my motive for this?" I ask that for our entire fondness for other humans: "What's our motive for this?"