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

Quite simply because life and death no longer depend on the quality of a vacuum tube, the way it did back in the hay days, during WW II, when tube technology drove design and manufacturing to the maxx.
We now just use tubes for a casual hobby, lives are no longer at stake, so there is a HUGE loss of military funding for vacuum tube design and manufacturing quality standards.

Cheers,
John

As Jmcgrogan2 mentioned, when military electronics, including radios, guidance systems and computers, were run on tubes, their reliability and endurance was paramount. Therefore, the tolerances, consistency and reliability of the tubes than ran the equipment were driven to the extreme. Hence, vintage tubes of the 40s, 50s, 60s and even 70s (particularly Eastern Bloc) were of excellent quality that cannot be matched today simply because modern military electronics don't require it, and therefore there's no large scale market to drive tube quality.

I think you guys are under estimating the importance to WE of reliable tubes and the fact that there was competition among manufacturers.

...and the fact that there was competition among manufacturers.
Tbg (System | Reviews | Threads | Answers)

I believe this does without saying. Every tube manufacturer would be competing to produce the best products to win the available military contracts.

I heard that RCA used the same machine to make tubes for 50+ years and every year the fittings got looser and looser so the newer tubes lost a bit of quality each year compared to the original tubes. I forget any more specifics but the general idea makes sense to me (in this instance). This is one part of the equation it seems...