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

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".

So mr_m 
your answer is “no, I have no data or evidence” 

Agree tomcy6. Some people prefer components that distort more (measure poorer). I was defining better as “distort less”.