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

Whether it is a dealer or eBay, there is a lot of marketing fluff.  The only way to know the life left in a tube is to have a transconductance and gas test.  That said there are many tube testers in use that are not calibrated and the results are not accurate.  

An old tube that is not correctly tested and sold as good could quickly go bad and take out bits of your amp and preamp.

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