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

You haven't found the right modern tubes then? Emission Labs and some of the special Shuguang (nature sound range) are superior to the best vintage tubes IMHO. But I guess you are stuck if they don't make the tube you need?

I think the vintage tube obsession, unless you cannot find a good modern version, is bogus.

Nearly all the vintage tubes I have had over the years sounded Dark, or metallic, grainy or probably too fat or overly saturated. Try the mesh plates from EML. They are wonderful (and expensive!) but the nature sound are different sounding but as good and probably a lot cheaper than the EML

Was was told once a while back by a 90 year old English tube guru who also worked in tube manufacturing back in the early days, I think he maybe RIP by now.
He said to me that todays tubes will never match a good NOS tube, because back then they used lots of Nickel in the manufacturing process, and today they don't so much if any as it is very poisonous.

Cheers George

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?