> Even used its almost always been above $250, with an average used price over $350.
It's a high end 1060. Which in practice means almost nothing but that people will overpay for it, expecting better performance and quality than they'd get.
The $400 price when last available is a pricing artifact of the end of cards lifetime. When they stop being available, some vendors will ask ridiculous prices to basically scam those that do not know they could get a newer better model instead, or are enterprise customers that have to buy the exact same model as the one to be replaced because of bureaucratic stupidity.
Also, camelcamelcamel tracks Amazon prices. US Amazon is the most ridiculously overpriced hardware shop I've ever seen, with only occasional good offers in between. You can't take those prices of specific models serious.
So yes, this 1060 might have had a "shortage" after it stopped being produced and even 1060s in general did have one at one point in time, but at the same time GTX 1650 Super, the GTX 1660, the GTX 1660 Super or especially a RX 480/580 had been readily available (after the crypto bubble of course).
Why multiple years? It's unlikely the current shortage will take that long, and it only started at the end of last year. In Summer 2020 buying a gpu was not a problem. Then the RTX 3000 series launched with basically no cards actually being available, and all following releases were like that from both vendors.
But both the RTX 2000 series and the Radeon RX 5000 cards have been easily available back then (okay, iirc the USA had some supply issues, but that was domestic Covid and not production related), so there was no reason for them to ramp up production for now during the last 3 or 4 years.