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

Because the old craftsmen died 50 years ago.HELLO!!!!

☝🏻☝🏻 True enough. A lot of craftsmanship, old school tech, and perhaps now-banned materials are not available. 

I use some "vintage" tubes...I like chrome dome Sylvanias and Mullards that "flash." I don't know why manufacturers keep putting KT150s, KT120s, and Gold Lion KT88s in the amps they build...don't they know new tubes suck? They even fooled me into enjoying all of those and thinking they sounded great...some for years...

Current tube production is aimed at a small niche market.  Before transistors, tubes ran our electronic world.  They were critical and manufacturer's had to make quality product for the military, government and..  We are the hobby market and really don't amount to huge profits.

+1 for Sylvania Chrome domes , the Goldilocks of tubes , not too little, not too much .