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

There is also a Japanese company - Takatsuki - that makes a few tubes. . . notable the 300b. Very expensive, but also very good.

New production tubes that I can vouch for
1) EML.
2) Psvane, particularly their top level W.E. Replica series.
3) Shuguang, their treasure series.
4) Takatsuki,
These can compete with the best tubes of any era in my opinion.
There are some very good NOS tubes to be certain but there are lemons in that niche as well.
Charles,

Charles. Which Psvane WE Replicas have you tried?

Gary,
I've heard their 845 and it sounded better than NOS RCA 845 in a friend's SET amplifier. I use the Psvane 101D Hifi series tube in my preamplifier. People who've heard both say the W.E. 101D version is significantly superior ( and the Hifi version is very good!).I haven't heard their 300b but again it's said to be quite good.
Charles,

I have to say we have had extremely good luck with the run-of-the-mill Chinese 6SN7. We've been using it for about 14 years. Its not the best sounding tube, but its not bad, and they've been pretty reliable.