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

Why aren't new vacuum tubes as good as or better than old ones? Don't we have purer metals? Precision equipment? Why isn't anyone making the highly regarded 7316s?
Thanks Wolf except MY experience has not been as good, at least the last two times out. I sure hope the third time is the charm. This time I purchased the one year warranty for an additional 20% of the tube price. The last two pair didn't last beyond 3 months, 6SN7 in a pre output stage. Maybe an unusual experience, or something wrong with my preamp, for sure not the latter, the problem is tube quality.

I test ALL tubes I purchase prior to installation and keep a log of the dates installed. In the case of the first two sets, when I noticed noise I checked them out and the transconductance had gone South considerably, not a short, the tubes just didn't hold up, one was marginal the other tested weak and replace. The manufacturer sent me another pair. These lasted a bit longer, 5 months but certainly no more than 1500-1800 hours and I'm being generous. When I installed the most recent tubes a few days ago I tested the old stock Sylvanias I had in the pre-amp that were installed a little over a year ago to replace the failed tubes. They tested stronger than than the new tubes I just purchased and the same as the day I installed them. Unfortunately these very nice Sylvanias don't come close to the sound of the Chinese tubes, it ain't a subtle difference for sure. For 250.00 is it unreasonable to expect at least 4K hours of life or more? Or is this the new normal or is my experience unusual?
I think you should avoid the 6SN7s from that manufacturer as clearly something's wrong...but preamp tubes or driver tubes generally should last a very long time. Also, 250 bucks is some large cash for 4 driver or preamp tubes...sort of my lifetime preamp tube cost, although I can see (at thetubestore anyway) 6SN7s seem pricier than some other preamp types.
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.