What's the potential benefit? What variables would you tweak on a custom bespoke keyboard?
Well I'm an Emacs user and would love to have Apple's butterfly mechanism style keyboard with the existing modifier keys for macOS (Command, Option & Control) plus the modifier keys from the Symbolics keyboards (Meta, Super & Hyper). That way I can separate out the key chords for window, application, and system commands from the key chords for Emacs text editing commands. I'd pay at least $1000 to have it custom built.
I think some of the variables include key layout, key switch type, keycap shape & material, key dimensions, the material of the keyboard itself, type of connector, LED colors & placement, extras like auxiliary displays, knobs, switches, sliders, and touchpads. I could continue...
In the world of guitars one can spend thousands of dollars having the perfect custom guitar built. I've often wondered why software developers don't invest as much in bespoke custom keyboards for exactly the same reason you have come to realize. I definitely think there's a market for it.