There certainly was a time (probably any time before the '60s) when a relatively large portion of society owned guns and shot (at least to hunt) but the vast majority owned one, two, maybe three guns at the outside. They had what they needed and didn't go in for extras, or collectibles. Even though the prices in those old ads look tiny to us now, guns were very expensive compared to average income levels. The "shooting sports" hardly existed and there weren't many people who would think of spending the family's money on a case or safe full of guns they didn't need.
Now that "most" guns cost less than a few days' pay for most people, folks tend to collect them, buy-and-try, and sell off stuff they don't instantly like.
Another thing to consider: Today we have pretty high expectations, consumer protection laws (and a culture of lawsuits), and a lot of competition in the arms world (coupled with better and better CNC machining) pushing high-durability products at low costs. The great old guns of yesterday that we remember so fondly are the ones that WERE great guns. But there were LOTS-and-lots that weren't. Pot metal, crappy throw-away pistols and revolvers from no-name makers in Europe, low-end single-shot shotguns, and lots of other junk on the market that probably wasn't even worth trying to sell once it broke. The lower-cost rifles that were around were often military surplus, so the relative quality was probably pretty good, but the temptation was ever-present to make up a poor-man's Winchester with a hacksaw and file.
All-in-all, I'd absolutely expect that what's on the used market now is better. You've got the best of the stuff that survived from yesteryear, plus a whole lot of rifles and shotguns from the middle of last century that got sold out of deceased uncles' or grand-dads' closets and can be had cheap, lots of police-turn-in pistols (and some revolvers, still) from the last quarter of the 20th century as the polymer guns started to take over those markets, and all the newer stuff that didn't turn gamer-Jimmy into an overnight shooting rock star the moment he got it home and out of the box.
It's not a bad time to be buying used.