In the 70s and 80s, when the vintage market got started, the older guitars from the 50s and 60s truly were better. It wasn’t a myth. They sounded better and played better than the 70s stuff (and I would argue, much of the 80s stuff, except for a few builders like PRS). Same with amps, by the way.
Later, a different, untrue myth developed: The idea that “older” = “better.” This happened among folks who didn’t know why the whole vintage market got going, which was to get AWAY from the 70s guitars; all they were after was being able to say the guitar was “vintage.”
Throw in the fact that most players haven’t ever even played a 50s or 60s guitar, only reissues. There’s no basis for comparison. The whole thing is an ignorance-fest of major proportions.
Thus the 70s junk got hot for those who didn’t get in on the 50s and 60s stuff before the prices went out of reach, on the myth that older is better, and to that you can add the folks who want an early pedal, a low serial number, and all that other nonsense.
Fact is, most folks don’t know the difference between a great guitar and a mediocre or poorly made one. Most folks don’t have any experience at all with a truly great vintage amp. And predictably, Fender has started to reissue Silverface amps (that of course don’t even meet that standard), because people don’t know that the blackface amps were in fact more desirable.
I’ll throw in another fact: not all 50s and 60s guitars were great. There were plenty of clunkers. A few people understand and can hear the difference, but many just want to be able to say or feel good about having “a ‘65.”
I’ve played some “vintage” jazz boxes from the 40s and 50s that their owners paid a pretty penny for, and some of them were utterly lifeless and sour sounding. But to have a “vintage” D’Aquisto Or D’Angelico from the 40s or 50s, or some such in your possession? Big status, even if it sounds like a turd, cracked nitro finish notwithstanding!
The world is full of suckers. So what else is new?