I was asking that question about why revulsion from torture isn’t universal five years ago, too, almost to the day. I’ll just repost it.
Lynn Hunt asks a pertinent question in Inventing Human Rights:
Voltaire railed against the miscarriage of justice in the Calas case, but he did not originally object to the fact that the old man had been tortured or broken on the wheel. If natural compassion makes everyone detest the cruelty of judicial torture, as Voltaire said later, then why was this not obvious before the 1760s, even to him? Evidently some kind of blinders had operated to inhibit the operation of empathy before then.
The facts aren’t enough. Science isn’t enough. There has to be emotion too. People have to care. It’s that simple. If people don’t care, the facts are just facts, they’re inert.
This is also why relief organizations use one person (and animal welfare organizations use one animal) on fund-raising appeals: we’re wired so that we empathize with one person much more strongly than we empathize with a million. If facts were enough for morality, we ought to respond a million times more strongly to reports of a million people in desperate straits, but in fact we respond much less strongly to a million people than we do to one.