Why do old tubes sound better than current tubes?

Yes, "new old stock" not only sounds like marketing gibberish but an oxymoron as well, which is why many prefer "new original stock", which is a more accurately descriptive term for the NOS acronym. This distinguishes the older production from the modern reissues.

I claim to have golden ears in regard to NOS tubes. I bought some bargain NOS tubes that sounded half as good as full price tubes and I sent them back for full price tubes. MY point is, leave bargains for the bottom feeders.
I have bought tubes from "Brent Jesse" for years. A reputable dealer makes his living dealing with "golden ears", he can not afford not to deliver the goods. Since there is absolutely no way to tell an OK tube from the real deal by looking at it, and only if you got the ears can you tell by listining to it; your best option is to only deal with a reputable dealer.

I found this and thought it interesting:

NOS

some old tubes sound great, some sound like crap, some new tubes sound great, some like crap. Sorry ,but the notion that anything nos is better than anything new is just not true. I listen to the sound, not somebody's marketing hype.

"some old tubes sound great, some sound like crap, some new tubes sound great, some like crap"

I suspect this to be true.