Something to consider, D&D was invented in 1975.
This was a very different time in many ways, I'll list a few
#1 It was culturally and racially much more homogeneous. we all saw the same three channels, heard the same pop music in our limited interest sphere and mostly saw the same movies and such. Books were where most of the differences were though many fewer were published than especially in Sci Fi and Fantasy
#2 It was an industrial society where men worked and most women didn't not a post industrial one were women are starting to outnumber men in the work force
#3 People matured faster and there were fewer socially maladapted gamers
#4 Vietnam had just ended and the US was undergoing major cultural turbulence
#5 There were essentially no personal computers and in fact up till the mid 90's it was plausible for everything in a campaign to be in a notebook.
#6 There was a lot less game material out there will well into the 80's . Even a mediocre issue of Dragon would be prized
#7 There was some Sci Fi (Star Wars wouldn't be out till D&D was 2) and very little fantasy, few enough that it would be perfectly plausible to have read the entire appendix N. Everybody knew the same stuff and there wasn't much of it.
#8 History especially material history in the US was laughable. Very few people had any idea about the past. A lot of people didn't really care though, they just ate whatever was in the book and played the game.
#9 D&D original designers grew up in the age of westerns which has a strong influence on D&D, In many respects D&D, especially Greyhawk is a post apoc western with medieval and magic trappings
#10 The psychedelic movement and such was still out there and it had an occasional impact on D&D and as I understand it, D&D was originally a stoner thing not a nerd game.
The Return of Planescape? - Curious. There is a brief note on this 'countdown' at Enworld.
15 hours ago