Why Can't Tubes Be Mass-Produced Cheaply?

I often read that old tubes manufactured from the 60's or earlier are collectible items and often much sought after, and new tubes are not as valuable. Reasons cited are that these older tubes are a rare species, and they often sound 'better' and hence the hefty price tag on them. I am puzzled as to how these older tubes are different from the new ones and why they are better-sounding. Why can't technology today produce tubes that are similiar in quality to those in the yesteryears, or even better them? After all vacuum tubes, electron tubes or valve tube(where they call it in Britain) are electronic components made up by plates and filament. What happens if these old tubes become extinct? Why are there still so many of these old tubes available for sale although it has been almost 4 decades since they were manufactured? I mean once these tubes have run out of life they will basically be disposed off.

Pardon my ignorance as I cannot seem to find any discussion on this matter elsewhere. Any opinions would be much appreciated.