I can understand it. A lot of the time their families assume the person went off and had their own life, not wanting to talk to them anymore. Families didn't like to think the worst and UIDs weren't really a thing that was well known. They probably assumed that if their missing family member was dead, they'd find out about it somehow. Families aren't easy sometimes and some people understand if a sibling etc wants to cut ties with parents and everyone else. Sometimes these people will try to look for their loved one years later but will still assume they're alive, searching online directories etc. It just doesn't occur to them that the loved one might have died decades ago.
Then there's cases where they have no idea where to report their family member missing. If they've been travelling around the country and you don't even know where they last were, which state do you report them missing in? Especially back then, lots of places wouldn't take a MP report from someone who wasn't even known to be in their state at all. Try to report them missing in your own state? Lots of police will just say 'but they left the state, it's not our problem anymore'. At least back then.
And then you have people on the very edge of society. People whose families don't really want them in their lives. Addicts, abusers, people who have done things to hurt their families. Theft, abandonment, alcoholism. People who have been given every opportunity to change by their family, given so many second chances, and still haven't changed. I know someone who has no idea where her sister is but won't report her missing because her sister is an addict, and if the police found her and said 'your sister reported you missing', she'd show up at the door trying to beg for drug money because it gives her the impression that she's open for contact.
And you also have to consider that a lot of the families themselves had such issues. Parents who were addicts, who were petty criminals, who had untreated mental health issues. It's not a surprise that they'd just think the kid was better off away from the family and wouldn't report them missing.
It's a lot more complicated than just not caring, I think, even though it is sad.