Why aren't new vacuum tubes as good as old ones?

"gotta scratch that niche" ..nice
It seems as if more and more tube products are now out there compared to 15-20 years ago so,
while still not huge in the market I do think it is growing
.AND,
as Wolf_Garcia  stated
"how many tube guitar amps are out there?".
quite alot I'd imagine.
So maybe there would be some incentive to MFG some quality Tubes eventually.

Here is stuff I have that needs Tubes;
Stereo-
Schiit Audio Freya
Primaluna Prologue 5

Geetar Related-
Dr Z Rt 66 Head
Egnator Rebel 20 Head
Egnator Tweaker Head
Peavey JSX Mini Colossal  Combo
Peavey 6505 Piranha  Head
Gibson/Kalamazoo Model 1 Combo
JetCity JCA20 Combo Combo
Blackstar HT1 Head
Orange Micro Terror Head
Orange Micro Dark Terror Head
Vox AC4T1 head and combo
Epiphone Valve JR 5 watt head
VHT Special 6 head [2 of them]
Blackheart Handsome Devil 15 watt head
Blackheart Little Giant 5 watt head
Blackheart Killer Ant 1 watt heads[3 of those!,2 modified]


So,that's around 22 things around the house that use Tubes.
A few are only using tubes in the preamp section and
 all the others are all tube
so I am all for increased MFG of quality tubes

I suggest you try what Billy Gibbons allegedly did with a bunch of amps...his crew piled them up in a sort of large box configuration and stuck a mic in the middle of the heap...not sure how it sounded but hey, why not? 

like  Marty in Back to the Future with a bunch of amps instead of One Big One

Can anyone provide data or evidence for the claim that new tubes don't sound as good?

Data or evidence would mean things like tube characteristic curves showing that old tubes are closer to the ideal than new or double blind hearing tests showing that listeners can easily detect the difference.
 
Data and evidence would not include unsubstantiated statements like "in my opinion ABC NOS tubes are way better than XYZ new tubes".