[SonderDesign]

I'm one of the founders, E Ink displays have their display drivers on their glass substrate, meaning you would have a border that on a single key display would be too significant. We went with one large display with only one driving board for the entirety of the keys. This greatly reduced the unit cost to have one large EPD than 80+ small ones.

[TheRealPomax]

And in the process designed a keyboard that is probably a little worse than apple's execution on this ergonomic nightmare. The challenge in this space was to make a real keyboard - sure, that'd involve overcoming the "how do we get the wires from the board through the switch to the keycap", starting with off the shelf switches and caps (because it's okay to not offer people the choice to replace their own keys when you make something like this) because as you say, it would be way harder (and far more costly) to make individual keys be self-contained e-ink units.