I too grew up in the in that era. But grew up in the Detroit area where nearly everybody knew something about cars due to how many people worked in the auto industry and how the knowledge was prevelant and shared amongst friends and peers. Auto shops were still a thing, but largely used for jobs nobody wanted to do, didn't have access to the tools, or didn't have the time to invest

It's true not everyone could do it, but things like checking timing, adjusting carburettors etc is something people can learn from reading a manual or watching a video.