Are all of these NOS tube inventories real? How have we not consumed them already?

Just like with old Roman coins, there are caches of tubes to be found and there are those that do the job of finding them. Your best bet is to find a reputable dealer and one of them is Brent Jessee:
http://www.audiotubes.com/
Take a look at the chart, choose your tube, and then proceed to read the history of the tube, the types available, and what he has in stock.

All the best,
Nonoise

There may be lots of NOS or NIB tubes around but try finding a truly NEW late 1950's early 1960s Siemens and Halske Cca 6922 grey plates or a late 1950's pinched waist Valvo Cca 6922.  Abot a decade ago a dealer told me NOS was anything that still tested OK, and NIB was anything that tested OK in any box.  Just look at the pins...  they tell a tale and beware fakes.  You can print just about anything on anything.

midareff1

... a decade ago a dealer told me NOS was anything that still tested OK, and NIB was anything that tested OK in any box.

NOS = New Old Stock
NIB = New In Box
Used ≠ New Old Stock
Used ≠ New In Box

To write off someone like Brent who's been at this longer than most and is honest as the day is long because one dealer told you something tells me a lot. Just like one can print anything about anything is like anyone can post anything about almost anything. 

Have you ever dealt with the guy?

All the best,
Nonoise

You really just have to do your homework with vintage valves or NOS. 
You need to know what labels, plates, and getter constructions look like, and be able to look up valid date codes. 
The vacuum tube industry was just as incestuous and precarious as the record pressing industry. Many brand name tubes were assembled at other tube companies factories and the tooling changed hands as well. There’s a lot to know. 
Often, someone like Andy at VTS is one who could tell you what’s what. Fore mentioned Kevin from Upscale and Brent Jessee are all good resources, but you pay for what you get. Many other folks are just talking out of their arses. Parroting what someone else has said on the internets. Crazy. 
If high quality signal tubes last 5-10k hours, it’s not much of an argument that they are a value even at several hundred dollars apiece if you find the right tubes, in the right condition, for the right circuit! But now, or cry later.