To inure is “to habituate to something undesirable, especially by prolonged subjection” or acculturation. If you are subjected to something long enough and often enough (e.g. spending time in slaughterhouses or jails or emergency wards or factory farms or “old age” homes or street gangs or torture prisons or refugee camps or ghettos or the armed forces or police forces, or living with an abuser, or watching violent “entertainment”) you become habituated to it. You become unable to feel the strong negative emotions and visceral revulsion that you would if this were a rare or brief event. You cannot. You emotionally detach, disengage, dissociate. No one can sustain that intensity of emotion indefinitely. The emotion gets suppressed, turned inward, and eventually the chemical reaction that occurs no longer has the same effect. You become emotionally flattened, numbed.
From the perspective of a massive human culture that is trying to get all seven billion of its members to work hard without anger, grief, outrage, or complaint, such emotional flattening provides a huge evolutionary advantage.
If you can be inured to not care, or to not care to know, you can be made to do anything.
Or, in the face of continued cultural atrocities, to do nothing.