It’s not that they’re that hard to make, it’s that they have low demand and the manufacturing process has some bad pollution side effects, though I’m not sure they’re that much worse than semiconductor manufacturing. They’re also fragile compared to solid state.
I think that basically guitar players and recording studios and high-end stereo owners don’t purchase enough tubes to keep the companies alive. Like all things vintage, eventually the prices for tubes will be high enough to encourage some boutique companies to make them, but what they will cost is unknown.
I had three boutique tube amps before I got my FM3 and FX3. One of the reasons I got the Fractals was I know it’s getting harder and harder to get decent tubes and I am not that interested in the escalating prices. My amp tech got a big supply of NOS tubes, so I had him retube my amps, then Cygnus came out and I decided to slowly step away from tubes and sold my biggest amp after pulling the tubes and putting in the originals. I am debating selling my other two amps and turning the money around to buy one last “forever” tube amp, and living with my own personal stock of tubes I’ve acquired over the years as backups, and use my FM3 system as my primary “amp”.
There are (were?) some solid-state tubes being worked on but who knows if they’re capable of replacing regular tubes for guitarist’s needs.
https://spectrum.ieee.org/introducing-the-vacuum-transistor-a-device-made-of-nothing