Why are older tubes so much sought after

Chadeffect, I hope you're right! but history has shown that you're probably wrong. As witnessed by the tube resurgence of the last 10-15 yrs. when obviously newer and better solidstate technologies have evolved, tube equipment is still thriving! So,obviously a lot of people still think dreadfully inefficient technology is fine.
One thing I have learned and you should've also, is that logic doesn't apply to highend audio.

Rodman99999, I'm using the RCA 7199. Sylvanias were also an option, but most people told me the RCA were preferable. Not having tried the Sylvanias, I don't know how they stack up against the RCA, but I was assured either would be a tremendous improvement over the Sovteks.

I'll need to try an NOS rectifier, based on your recommendation.

As far as new production tubes go, there are several I like, the JJ KT88 and E34L and EH 6CA7 among them, along with the Shuguang 12AX7. But, apart from those, I come down on the side that the old tubes absolutely smoke current production.

I echo all who said that they simply sound better but are at times in poor working condition. The NOS tubes are virtually gone except for the JAN military surplus made in the 70s and 80s believe it or not The occasional treasure found in a repair man's attick or tubes nobody wants.
What you find are used tubes that test well on a questionable tube tester because that too is over 40 years old. Yes I know you can get it to spec by sending it to a pro. My experience is that tolerances drift easily with these old (explective).
In essence I have learned in my collecting tubes that it is truly just a hit or miss on ebay but with dealers you get to know, they will sell you only tubes that they expect to hold to spec longer than other thus "good" tubes. There really is the occasional good tubes is all I sell dealer but that is rare.
Old stock tubes are also often superior because of they afford you variety. Tubes in their hey day came from many many brands, that had there own recipe, to actually give their tube a certain flavor e.g. "Our tube is better than ordinary tubes find out and you will only smoke Mazda tubes." (All tube making facilities secretly were and are owned by Phillips. Yes I know that all brands were not made by Phillips and please I am not bashing Mazda) thus you get different sonics with the brand you buy. So it gives you a tone knob on our very simple path amps and pre-amps.
However you still need guidance from a person who truly knows what that particular tube sounds like compared to others.
For example one can make generic statements about a particular brand such as Mullard such as " they are warm colored." Simply put this person is correct, but not all the time. I use a military variant for a 6922. This tube is made by Mullard and is a drop in sub. This tube is not warm and fuzzy it sounds very fast and very clean with a powerfull voice comparable to a German tube. ( The Mullard CV 2943s)
Trelja I have a few pairs of 7199s but all brands are made by Amperex no matter what the label is. I think so anyway. It could be Sylvania I have to find out. Non Trelja people please do not write asking for these tubes -thanks

Trelja- I gave my son my ST70 some years back for his birthday. He ended up with RCAs as Sylvanias weren't available when the ones in it bit the dust. Everything else in the system was different, but- They sounded like an excellent tube(especially the imaging and soundstage). My Cary monoblocks take 6550/KT88 output tubes(matched quads necessary), so- nothing NOS is actually an option regarding price. I'd have to sell a couple ounces of gold to purchase the EAT KT88 Diamonds right now, so- I'm using Svetlana 6550's(latest iteration, ST Petersbug plant) from Upscale Audio. I was really pleased(shocked/surprised) that they actually peformed as good as the NOS Phillips ECGs I had in the Carys. I tried some other current production tubes in them first, and couldn't get them out of the amps fast enough. Off the subject: If you want a quantum leap in the performance of the ST70- Upgrade the coupling caps to film/foil, and install a better power cord. I used a Zu Birth without having to modify the chassis in any way. Those were just two of the basic things(among a host of others) I did to mine that REALLY yielded major results as far as accuracy.

We seek out old tubes because the Gilbert Erector Sets, Tinkertoys, Lionel Trains, and GI Joes so popular in our youth are more expensive and difficult to find in mint condition.

And they're cheaper to ship.