Why are NOS tubes considered superior?

The idea of "tube rolling" in my ARC preamp is something that I am considering. What I'm not sure I understand is the quality of tubes manufactured today vs. older tubes. It would seem that with modern manufacturing technology new tubes would be of higher quality than tubes made in the 60's and 70's. How are the older tubes better?
The older tubes are better IF you have a reputable supplier that properly tests them. Some of the tubes are less microphonic, some just sound better because of the midrange bloom. This is a fine example of tweeks, but I have done this to my ARC gear with GREAT results. If you would like to know my tweeks, write me off line.
Some NOS are better, some are not. Know what you are buying. Many current manufactured tubes are the best ever. Sometimes mid-range bloom can be achieved with NOS tubes because of inherent problems with equipment and equipment matching. Just because a tube is NOS puts it far from a recommendation. Good luck.
NOS tubes are highly sought after by guitar players for our vacuum tube amps. We use a lot of tubes. There are web sites that specialize in NOS tubes. Mullards,Telefunkens, Brimars, RCA's and GE's sound dramaticaly superior to new Soviet and Chinese tubes. The Russian tubes have been improving and are "good enough" for most uses. The Chinese are mixed; many are microphonic and fail after several months. It has to do with the mechanical assembly and the rare earths used in the tubes. The factory skill set that was lost when the West shut down the production lines has not been recreated. Perhaps it never will be. For now there are good stocks of JAN (military)and old TV/Radio shop stocks to be worked out.
In nearly all my audio gear certain NOS tubes have been superior to those fitted by the manufacture. So many have not been making audiitoning a neccessity. If you develop a relationship with some of the NOS tube suppliers, most likely opportunity to experiment will be offered. Interestingly, in my CAT amps the Telefunken 802s, which were spectacular in my Jadis amp,have exhibited no improvement. Actually, a heavy, dark signature has emerged. Take caution with any investment and allow your ears to decide whether to replace or reject. Good luck. This can be a fun part of tweaking a system. The sound will efinitely change, especially if your system is revealing.