Have those people built the keyboard themselves, though? I've been periodically checking the more popular/accessible low-profile keyboards and other than Keychron and Logitech it was difficult to find something available in Europe. Low-profile switches definitely exist, but keyboards are pretty rare. If you add requirements like USB-C and bluetooth, there's really not much choice left (or is there? I'd love to be proven wrong)
Choice does not equal demand. Low profile switches are relatively new. And no, there are a few brands that build them. I would not build a board myself.
