I suspect that this is because there has been very little progress in amplifier terms since the implementation of transistor based circuitry 40-50 years ago. Sure, there have been developments but no step changes. The same basic circuits are still in use, maybe with the introduction of a regulated PSU here and a few noise reducing caps there, but still the same old circuits. 20 years ago when I discovered vintage stuff you could buy Quad 303s and 405s for loose change. Early Naim was cheapish too. I went the Quad route and had dazzling results for loose change. Can you believe a 33 for £30 and a serviced 303 for £60? Nor could I, when I heard them. Unplinthed Garrard 401s were £75-100, SME 3009s £50-75. They were just as good then as they are now. This is because the manufacturers still had the magazines in their pockets and it was in the industry's interest, in those pre-internet days, to maintain the belief that the newer, shinier item was always better than last year's and that any old relics from the 70's or (heaven forbid) the 60's were, well, of historical interest, but really rather dated now.
People have now had 20 years of the internet and the cat is out of the bag. The step changes (CD, internet streaming) have been adopted, while the stuff that was more or less perfected by 1970 is still worth using and, properly serviced, it's very good indeed by any standards. A Garrard 401 or Thorens 124 with an SME 3009, going through a Leak TL12 or Quad 2 into Tannoy Reds/Golds or Quad 57s is still world class. It shouldn't be, but it is. Then again, look at the current manufacturing costs for a Garrard/Quad 2/ESL57 and your eyes will water.
I'm interested to see what can be made from the current breed of chip amps. These offer dazzling performance per £, and they do make the tired old circuits of yesteryear look rather dated.