It can be a symptom of ilness
When I worked as an IT supporter, there would sometimes be calls from not so sane people, and their reality just looks very different, and you can't really say anything to them that would comfort them, like you can with a a sane person, since they do not connect what you say.

they get so focused on their own reality, they can't break free.
And the complete lack of empathy, as you said, seems to fit very well.