Good question, I believe it's because 'they don't build 'em like they used to' philosophy. In business today, it's all about the bottom line, getting the fewest amount of people to do the most work possible. In the older days, the quality was priority. Now it's how to make it cheaper, so the bottom line looks better. Profit margins are the drivers today, it wasn't like that BR (Before Reaganomics). Business' are run like there is no tomorrow, make the most you can TODAY.
NOS tubes, why can't modern tubes sound better?
Perhaps this is a useless thread, but why can't currently manufactured tubes at least equal the sound quality of NOS? With the advances in science, why is it that a modern factory cannot exceed the sound quality of the past? Perhaps for the same unexplained reasons that modern violin manufacturers cannot attain the 'Strats'.