Because people DO care, but they generally don't show it much. Often it takes a shock like this for them to realize how much they actually do care.

It's easy to fall into the thought trap of "nobody cares", followed by "because there's no reason for them to care", to "because I'm / they're worthless", to "Nothing but parasites", to "We should all just go extinct."

We are all social beings, and deficiencies in social interaction can and do affect our psyche in very dramatic ways.