A variety of contributing factors.
Not so long ago, in my perspective, a lot of people did not have Social Security numbers. (That said, as time went by, more and more of the UNKNOWN did manage to show up in some records, now digital databases.)
Some rural deaths especially, might never be reported. For example, an older aunt or uncle on a struggling farm who are not participants in operations but in retirement; only the local community would know that. A later census - carefully examined after the fact - might catch, thru some scholarly attention, the change at a residence.
There are probably some number of people who have a “dependent” and fail to notify some administration of the dependent’s departure. (There was a recent story in the news, 2 weeks ago?)
Some old people are lost and forgotten. When I was young, I was amazed at how many people in articles and history that I read, had disappeared/vanished without a trace.
Media and TV portrayals of end of life, are not encyclopedic.
Lastly, the problem of databases. When I was in early employment, “my” company had one of the first generation computers that ate very-large-panel, punched (for edge-guidance thru machinery) printing paper like candy. The machine had a full-time staff (paid by the computer maker) to hover around and make that thing work.
Computers were rare. Healthy, large corporations could afford them, but acceptance at the county level was un-common.
The increasing acceptance that I saw of electronic digital databases, roughly matched the increasing issuance and usage of Social Security Numbers. Late 1950’s to about 1962, still saw people with no SSN; but by late 1960’s, certainly a majority did have an SSN. One of the fruits, probably, of economic growth after WW-II.
The country had much less in regulations, and people’s habits were not nearly as tuned to bureaucratic flow in terms of digitally recording data . . . as “the system(s)” is(are) now. What I mean, is, that there was not, especially in the countryside, a sense of having to comply with “the [new] system(s).”
PS. Average lap speeds at the Indianapolis 500 are TWICE what they were when I was a kid. And, I knew Pete dePaolo, the first guy to win the Indy 500 at just over 100 mph.
Welcome to FR and thanks for the detailed info.
I like to wander around old graveyards; especially the ones out west.
Many times I’ve seen family plots with the birth dates of all listed, but with many that have no death date.
I have assumed that many of these folks simply moved on to greener fields and died and were buried elsewhere.