Look at the size of the keyboard on the iPad Air 4 with its 9.74" height. Now imagine needing to shrink that down to fit the iPad Mini's 7.69" height. Those keys would be far too small to be usable by most. My Surface Pro keyboard is larger than the iPad Air one and still is smallish.
This complaint against the iPad Mini 6 is bogus at its core. This small form factor tablet is not intended to be a text entry device, at least not by typing on a tiny keyboard. If you want an all around, all purpose tablet, get a bigger one. The entire point of the iPad Mini 6th generation tablet is to have a stylish and highly portable media consumption or display device. If you must, however, enter text, you can always pair it with it with a folding Bluetooth keyboard or use voice to text input to write. There, problem solved.