In general, in electronics, when a firmware is involved and communicates with a higher-level software, it's almost always better to make the firmware as simple

In general, in electronics, when a firmware is involved and communicates with a higher-level software, it's almost always better to make the firmware as simple as possible, as dumb as possible, because the firmware is more difficult and tedious to update than the higher-level software, and it makes the hardware/firmware simpler and cheaper to design. For example say we followed your suggestion and the keyboard had to be responsible for key mapping. This means it would probably need non-volatile memory for saving the keymap in use (or it would forget it when unplugged.) The extra component adds a bit more cost, and increases the chances of hardware failure. If you want to forego NV memory, the software would have to be responsible for initializing the keymap every time the keyboard is plugged in. But if the software is choosing which keymap to use anyway, then technically it's simpler to let the key mappping happen in software and not firmware.