Why do you care?
Well, my question is, "Why don't they care?" But please note that this is an issue that I've had to deal with in several ways, from childless aunts and an uncle (including a pair of never-married aunts now having trouble taking care of themselves who now lean heavily on myself and my two cousins when things go bad) and a friend who recently died young and childless, with two childless siblings, whose family has given me first dibs on whatever I want to take from his belongings, some of which I want to try to keep in his family via a little cousin that he liked if that part of his family wants it. In addition, I recently talked to my father-in-law about one of his co-workers who, childless and facing failing health, killed himself with no obvious heirs. And then there were the years visiting my grandparents in a nursing home, seeing old people (including those with children, sadly) who never got visiters and were desperate to talk to someone -- anyone -- walking down the hallway. So I don't know about you but I'm seeing this starting to happen around me and find it a bit heartbreaking, especially since one of the great benefits of childlessness is supposed to be the extra money that people can spend on themselves, spending that will ultimately wind up in the ground with them (experiences) or in a dumpster or on Ebay (items). Consider the third ghost that visited Scrooge. Contrary to all of those Baby Boomer retirement account commercials, most people won't be windsurfing through their 80s.
And please note that I'm not saying that one person not having children is a bad thing. My childless aunts and uncle have nephews and nieces to leave their things to and some people become honorary members of another family. But the overall lack of children and smaller number of children means that only children (I'm one) don't have siblings to give them nephews and nieces and you won't get nephews and nieces if your siblings don't have children, either. Sure, some families are dysfunctional and some people find adoptive families but it's not the individual cases that are a problem so much as the overall pattern. One person walking on the grass isn't going to leave a rut but thousands are. One person deciding not to have children isn't going to wreck society (there have always been people who don't have children for various reasons, including an inability to do so) but large numbers taking that route and a birthrate well below replacement just might.