The reasons are myriad.
HiFi in the 1970s was aspirational, expensive in relative terms, exclusive and very much the centrepiece of a modern man/couple's entertainment space. The fact that advancements in technology were happening very quickly meant the industry was enormous and the Japanese were showing the world their expertise in precision, miniaturization and performance improvements.
People who lived in that era as young people, can now afford the gear they wished they had bought back then. Many of them were paying mortages, putting kids through school and now they are in their 60s-80s and have plenty of money to buy what they want. Supply and demand. The original stuff is mostly in landfill so the mint gear is worth a fortune. I know, I've been repairing and restoring this gear for decades, long before anyone cared for it. I kept telling people the TOTL vintage gear was going to be future 'gold'. It now is. A good Pioneer SX-1980 is easily worth US$7k. A Spec1/2 pair is worth AU$12k.
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There's nothing remotely interesting about modern/obsolete AVRs as compared to say an Pioneer SX-1250, and SX-1980, a Marantz 2600 or many of the midrange receivers where the battles for people's wallets took place.
By the time 1990 came around, the traditional HiFi receiver was morphing into a platform for guaranteed obsoletion every few years. Surround decoding systems appeared every other year. More amplifier channels, video/SVHS/Composite/HDMI v1etc connections. Everything is cheap plastic, poor construction, poor performance and not much more. I often pick up high range obsolete AVRs for a few dollars, not to use, but as a supply of NLA parts.
It's no accident that Yamaha resurrected its much loved vintage look about a decade ago- the original designs were by a Japanese design house that won several good design awards and was iconic. Marantz even have tried (poorly) to evoke the look of their past products. The trouble is, people want the original gear, not a modern take on it. They yearn for a simpler time, when you had a turntable, an AM/FM tuner and maybe a few other sources that always worked. You turned them on and they just worked. No menus, touchpads, internet, dropouts, reboots, crashes, glitches, bluetooth or phones.
2 channel HiFi survived barely during the AV/HT onslaught. But survive it did, and it came out the other end stronger and more in demand. Vintage receivers are part of that. I know plenty of guys with esoteric six figure audiophile systems that also have a few systems where handpicked, fully restored vintage 2 channel makes up the setups.