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

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 .

How many tube guitar amps are out there? Small niche? I'd bet they're 90% of the tube market...gotta scratch that niche.

Yes but current tube production is not critical to the communications business, like in the past. Tube makers could do better but where is the motivation.  Product longevity does not keep the line moving.  

"gotta scratch that niche" ..nice
It seems as if more and more tube products are now out there compared to 15-20 years ago so,
while still not huge in the market I do think it is growing
.AND,
as Wolf_Garcia  stated
"how many tube guitar amps are out there?".
quite alot I'd imagine.
So maybe there would be some incentive to MFG some quality Tubes eventually.