Requires a bigger chip (100+ IO pins) and more complex wiring diagrams than most inexpensive keyboard makers are willing to invest.

Replace with _any_ keyboard manufacture.. Similarly, while your keyboard may advertise USB2 or even USB3, the actual key-press USB interface is always running at USB 1 low speed (1.5Mbit).

I spent a fair amount of time trying to find a keyboard to work on a device that I have that only works with high speed devices, 30 or 40 keyboards later I gave up... If someone actually knows of such a thing I would be interested. Same basic problem with mice.. I guess the thought-process is that hey USB2 supports split transactions, and the keyboard/mouse won't actually generate even 1.5M bit of data, so we are going to continue to sell the same basic mouse/keyboard interfaces we were selling 20 years ago wrapped in styling variations.

PS: Some of the physical button keyboards I found with configurable colors/etc, usb hubs, do support USB3... For the color controls, or hub. The keyboard endpoint is still at low speed...

Likely because they want to implement the minimum necessary HID spec (or, in a nicer tone, the HID spec with the most compatibility), which would be the one supported by the BIOS.

I don't understand why someone hasn't come out with a dedicated keyboard chip yet. If it's cheap enough you don't need to run all the keys to a single chip, you could have multiple chips that all talk over a serial bus to one that it designated the master.

Another layer of serial interfaces would make it even slower.

A serial interface can run a lot faster than anyone can type, eg i2c high speed is max 3.2Mbit/s.

SPI can be way faster than that I believe.

true, but adding another link (microcontroller) in the chain is going to add delay anyway. I think the original suggestion was pointless: instead of adding more microcontrollers you can just replace the main uC with the one that has enough pins. The reason this is not done is uCs with >100 pins are usually more powerful and expensive, so you can't just pay for more pins -- you also have to pay for more processing power and features you don't need.

Depends on the speed/latency of the interface. Although you could probably say the same about the scanning matrix too.