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

would agree with @oldhvymec, at least on the preamp and low signal level tubes. There were tons and tons made. When tube production shifted from the US and Europe to Japan, many back then thought that was it, so hoarded US tubes by the boat load. Last year was out at RMAF. Ran into a guy and started talking tubes. He pulled out his phone and showed me pics of his 10 ft by 30 ft storage locker packed floor to ceiling with NOS tubes.......was something to see. 

Note that new tubes, or "currently manufactured tubes" for those who are seemingly confused about what NOS actually means

NOS and NIB are descriptions that only apply to OLD STOCK tubes. These terms shouldn’t be used with current production tubes. All of those should be unused and come in a box.

Many current production tubes use the names of the premier tube companies from the past like Mullard, Telefunken, Genelex, etc. All the current production tubes have in common with the old stock tubes are the names they have licensed from whoever owns the rights. The materials and construction are very different.

I’m not knocking current production tubes, Many people like the sound of current production tubes for small signal (preamp) tubes, and they are about the only type you can get for power tubes.

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.