The Golden Rule

[Meatball]

Pack mentality might help explain it. Human brains aren't wired to treat everyone equally. Just like a pack of monkeys might care for each other and protect each other, but attack a monkey of another pack.

Related: Dunbar's number

Also keep in mind that brains don't work in black and white like you suggest. For example, our brains tell us when something hurts by creating pain and pain memories to discourage. That doesn't mean we won't pull out a splinter even though it hurts more than leaving it in, or that we won't walk on a broken ankle to get help.

Things like morality and pain work as an influence, not as a rule.

Edit:

Also keep in might that brains don't work in black and white like you suggest.

I hope I did not suggest this. If I did, it was not meant. I was just trying to find out an atheistic position on the matter.

You didn't, I just read into that way. It's like asking why it's easy to forget phone numbers. I mean, our brains are built to remember things, right?