That's because the north, in particular new england, started industrializing in the late 1700s. While the south, due to a variety reasons, didn't.
> Slavery absolutely did not benefit the masters
If it didn't, the civil war wouldn't have happened. The wealthy elite who owned slaves benefited immensely. Just visit a plantation turned historical museum in the south. Most southerners didn't own slaves.
> "I'm better than you" is a human instinct that continues unabated to this day -- many absolutely adore that feeling as if it was a mind-altering drug hit.
Indeed.