Mostly because there’s not enough money or interest in creating a robotic assembly for tubes. Outside of auto manufacturing, full automation is exceptionally expensive. New tools can help create tubes more efficiently, but human labor is often still less expensive than total automation.
It’s a bit like a vintage car. Sure, we can make things that go faster, have launch control, computerized torque vectoring, even cars that drive themselves. But for the enthusiast, something about direct throttle cables, simple hydraulic power steering and slimmer, suspensions that let them feel the road is more immediate and satisfying.
And while we could make a Jaguar E Type or a Doosenburg or a Stingray Corvette with automation… the money and will to do so doesn’t exist.
I don’t know what the economics of fully automating tube production would look like, but the process is extensive and complex. It’s not as simple as blowing some glass bottles and sticking wires and grids in it. Vacuum sealing, furnace baking, chemistry lab/fume hoods, not to mention stricter environmental and safety regulations that make tube production difficult in North America’s remaining industrial areas are all considerations. Then, winding grids, hooking up the delicate wire to pins, sealing and testing them, all while trying to retain structural and electrical integrity of the piece. Double the cost and complexity as the tube gets smaller and adds additional grids or complex spacing arrangements.
Computer aided design is helpful, as you can digitize the tube drawings and do 3D modelling to determine structural weak points and assess tolerances. This requires someone who understands how a tubes design effects the sound of the tube though, and though I’ve met several tube factory owners and some incredibly talented engineers who can answer parts of that question, I can’t think of a single one who can sit down, draw up a tube schematic and have some idea of how it will turn out. They may be out there, but as for determining why different tubes of the same type can sound so different? Materials science, build tolerance, coatings, metallurgy, manufacturing expertise (such as the tightness of vacuum seal or the precision of grid winds for example) will likely all effect the sound of a tube. As for exactly how they will effect the sound, that’s a dark art that is beyond my understanding. I’ve heard many compelling theories for different tubes and techniques, but I couldn’t say I know whether I’m hearing the type or gauge of wire used in a grid wind, or if a graphite coating really does effect the sound in X or Y way. It’s still an exceptionally special engineer/listener who can say when ‘technical factor A occurs’ then ‘Sonic characteristic B emerges.’
I have myself added an iTube2 tube preamp just recently, and find I prefer it with some music but not others. Blues sounds even better, but a certain female vocal loses the “edge” that makes me almost want to send her a fan mail and propose. It has to do with control of the higher frequencies, and for her voice, they lose just a tad too much for me to go ecstatic. The enjoyment of the blues, though, seem to stem from something else, probably low-order harmonics. So my guess is that my ideal would be a wide FR tube with some harmonic distortion.