I wonder if the higher prices paid for these UHRs are a function of the non-collector interest in them plus the fact that so many of the coins have been off the market for decades. Many have been unable to be traced.
Years ago, coin collecting was mostly middle-aged to older people with $$$....they'd buy a coin and then once they tired of it, they'd sell it or let it re-circulate into new collections. Today, with these buyers of NFS and other social media and digital billionaires.....they have the $$$ to buy these coins in their 20's and 30's.
If they do buy it, unless they want to sell it or get tired of it....it could be with them for 50-60 years or more !
Not sure other key date-like coins in other coin types generate the same interest, but if they are rare and famous, the same problem can happen if the buyer is very young or just lives to a very old age and they don't want to sell it. But I don't think that's a concern with the special unique rarities among Franklins, Pennies, Nickels, etc.