As for settings, D and D was before Greyhawk, Blackmoor, Forgotten relms and so on. Traveller gave you rules for generating planets, subsectors, and sectors in the basic set before Supplements were produced. You were expected to make your own Imperium. Later it was filled out in publications or farmed out to judges guild, game lords, FASA ect.
The diference between gaming today and 35 years ago is DM's today expect nuts and bolt settings. Back then before the flood of aftermarket companies we made it up ourselves.
Agree entirely with this part.
I'll also point out that the rules in D&D, T&T, Starfaring... all of them left the underlying "reality" up to the GM, covering mostly combat and paranormal elements. CT defined a bit more, but the materials defined in 1977 really didn't define much: Ships, worlds, combat effects, psionics. All things beyond the average college student's experiences, even then.
If there is an underlying reality, it's a comparison of the survival rates to those of the US Military during the Vietnam war. (At least, they're within rounding error to 2d6 throws.) But does that imply an underlying reality being simulated? NO! it implies that Marc grabbed the then readily available data and adapted it for a general consistency with the target audience's expectations, rather than constructing an underlying "reality" to simulate/emulate.
Essentially, CT is assembled by "Rule of Cool"... as were most RPG's of the era. The exceptions being EPT and RuneQuest. EPT and RuneQuest both had underlying realities they were actively trying to emulate. That changes in late 1978... but the core rules didn't actually change to reflect that setting that was associated with them. Mind you, the core rules WERE changed in 1981, but not to be more OTU-friendly. They were simplified a bit in the combat department, and a few changes were made in healing.