That's almost impossible to answer since there are many different causes or reasons for other people's apparent or real lack of care...
Fear of consequences:-
They may have their own issues.
Be in denial about something.
Afraid that getting involved (as they see it) may have negative impact upon themselves.
Themselves be stressed out and struggling.
Reacting rather than responding (i.e. subjective rather than objective).
The stigma seems to be self perpetuating.
Lack of empathy (sociopathy). And sympathy is rather over-rated (my opinion only) since it seems to include a variable psychological/personal distancing factor...
Upon reflection, I'm not sure that trying to enumerate or list things is actually a useful thing to do. It doesn't seem to lead to resolution, now that I think about my own experiences over the last 50+ years or so...
Am I capitulating? No. Given that opinions are like bumcracks, everybody's got one and some are shittier than others. I try to keep my mind open and modify my own thoughts depending upon what I learn and experience.
I therefore submit that this is an extremely complex subject that does not lend itself to simplification or wholly logical analysis. Logic being hugely useful but can sometimes be simply a way to err with confidence.
You can no doubt see my own ambivalence and uncertainty.
Take care and good fortune each :)