Thermionic Labs makes a series of transformers specifically for headphones.
https://www.thermioniclab.com/en/headphone-transformers/ as does Van derveen
https://www.mennovanderveen.nl/index.php/nl/producten/other/vdv-800-v-hph-detail
Looks like it would be easy enough to have a selector switch to allow very different headphones to be used. I would also think you could use much lower powered/less expensive tubes since you can properly match impedance. I’ve always been puzzled by the use of such high power tubes like the el34, kt88, and 300b just to drive headphones. I would think these would be ideal but I haven’t seen anyone using them yet.
I'm not sure whether you're stating you have haven't seen anyone use these particular headphone transformers or whether you haven't seen anyone use headphone transformers, period. The use of output transformers on amplifiers have been used since the earliest day of electronic amplification. That includes headphones, too. There are many mfrs that make headphone specific output transformers and many mfrs will make custom transformers, too - with as many output impedance choices as you want.
The transformers you reference are interesting, but really don't present anything new, IMHO. A choice of four output taps vs. the "normal" two that are usually available for headphone use is interesting, but has its own consequences. It will require the use of a 8P/4T switch or a rotary, or a bank of DP/DT switches (with complication to prevent double/triple switching). From a circuit standpoint, it adds an unhealthy amount of complexity and expense to an already expensive choice: the use of output transformers.
That is the answer to your question. Not everyone designs an amplifier circuit to be expensive, which is what a "quality" output transformer choice would require. There are good output transformers and then there are great output transformers. The difference between the two can be extremely expensive. That's why a lot of people opt for the OTL topology. The downside is in order to get a reasonable amount of power from tubes without output transformers, you need high-power ones.
In practical terms, once you're talking in terms of double-digit and triple-digit output impedances, the differences between 16 and 32 ohms or 300 and 600 ohms are often negligible. Many find that the optimum solution (that's least expensive) is to simply use a low and high output impedance choice, or to design the amp for the headphones you wish to use, even perhaps for a single specific headphone. Even so, you can still get outstanding, world-class results simply from using 32/300 output transformers, if the transformers are made with outstanding materials.
