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

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

My Yamamoto A08s uses 45 tubes.
I have both the original WW2 era NOS set, hand / ear picked and matched by Mr. Shigeki Yamamoto himself, and a set of new production EML mesh 45s.
The EMLs are 3 leagues above the NOS in terms of sound quality in every aspect.
Yes, they do cost > $700 for the pair, but they are worth every penny :-)

I have Mullard NOS tubes from England. These are FAR better than the newer tubes that came with my amp. Not even close....

.... b/c back in the 1940s to the 1980s, the quality of the tube was literally a Life/Death situation during the COLD WAR and all the millions of pieces of military gear from complex communication systems to airplanes (as noted, for MIGs to withstand the Atomic Cloud after an A-bomb goes off--> since the transistor designs could NOT withstand the Atomic Cloud and would fail ... mid flight in the MIG).

Note all the later model cars running around with burnt out lamps (not LED's, a different story altogether regarding solder joints and high temp LED's), and yet I bet there are 1958 Chevy's with original bulbs all around the car. I never replace the old copper bulbs in vintage units if they are still good, as they will outlast any new bulb.

I have heard some tube types say the "quality" of the parts/spotwelds and the purity of the vacuum itself is not a good as it was before Solid State. Could be the same with today's lame ass bulbs.