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

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".

jyprez,

You don't need the data, all you have to do is listen to NOS vs. New.

"new vacuum tubes are not as good as old ones "
That used to be my line of thinking. NOS tubes were made better, better choice of materials, more consistent, etc.

After trying the latest PSVANE Treasure series, not sure I would agree with that original line of thinking anymore.

The sonic qualities on the T-II PSVANEs are so good, I actually prefer them to many NOS types.

this would include input tubes as well as rectifiers. The VANE 5U4G, is an excellent sounding tube,....