A common microcontroller board for building hobby mechanical keyboards is the Teensy 4. This uses an ARM Cortex M7 with 1MB RAM, 2MB flash, running at 600MHz.

A common microcontroller board for building hobby mechanical keyboards is the Teensy 4. This uses an ARM Cortex M7 with 1MB RAM, 2MB flash, running at 600MHz. In 1986, an exceptional machine like the IRIS 2000 ran at 10MHz, and fully-loaded had 16MB RAM. Do you need something as powerful as the ARM Cortex M7 to build a keyboard the way you're describing? Well, no, but you'd have trouble doing it with less than a 6502 or a Z80, and computer keyboards have existed for decades longer than those chips. Which were the entire CPU of microcomputers such as the Apple // and Commodore 128, respectively. So it would be possible now, but it would require rearchitecting a basic system which is working fine, for every operating system which could conceivably use it. Furthermore, most people would be inconvenienced by this. The two sides of the coin are, on the one hand, you plug in your mechanical keyboard into any computer and it Just Works. On the other hand are people who switch keyboard layouts. They outnumber the mech enthusiasts by what must be hundreds to one, switching layouts based on what one is doing is very common in Europe and ubiquitous in places where the language is written using a script which isn't derived from the Latin one. So that's the historical reason why it didn't happen, and the contemporary reason why it still hasn't, and probably won't. This actually inconveniences me, I have to jump through several levels of hoop to put Unicode characters on the higher planes of my split-key, but it is what it is.