Richardson Electronics has been making some, I believe copies of the MO Valve KT88s and the WE 300Bs, and the Chinese Golden Dragons were supposed to be exact copies of the MO Valve KT88s. Richardson's are or were exorbitantly priced, due to the excessive tooling costs and limited demand, and the reliability of the Chinese tubes has been questionable, to say the least. In addition, I believe that EI of Yugoslavia had the old Telefunken equipment, and produced a number of 12AX7 and 12AU7 type tubes, among others, until the war there apparently destroyed their factory. Those tubes had some, but not all, of the Teles' magic; I have some in reserve if I run out of Teles. I think Ed is right, unfortunately; it's principally economics, as well as part execution and part materials. On the bright side, a lot of the new tube equipment has been designed around currently available tubes, and often "voiced" with those tubes in mind, so that you can still get excellent sound from current production tubes.
NOS tubes new production
I was wondering why some tube manufacturer or small niche manufacturer doesn't make tubes that are exactly the same as the rare type of NOS that are in such high demand and sound the best. The manufacturers of the old Seimens, Telefunken, Mullard etc made the best tubes. The new production of these models from other manufacturers don't always sound as good as the old stock. Would the demand be too small to make it worth while? Are the patents a roadblock to this problem? A good example of this is the EL34 where the old stock sounds the best and the prices are skyrocketing, or the 7308-nothing new sounds as good as the NOS. I know that it can be done technically. Would this be too expensive? We need to pick the brains of these old guys that manufactured them before they are all gone. I know that there are supplies of new old stock available from ebay etc, but this is not the same as going to a dealer to buy new tubes that have a guarantee and that you can have confidence in. I have bought tubes from ebay that are NOS, but with my experience I wonder are these used, or is the production date code the ones that are not as good or are these fakes or relabeled? Just the hazards of NOS. This problem will only get worse as they get harder to find.