The reason is, let’s say you have an improper ulnar deviation when you type (wrist twists to the left on your left and right on your right), you can still execute that improper motion with an ergonomic keyboard and, what good is fixing your typing deviation when you open doors, brush your teeth, write, use your phone or drive your car with the same deviation?
You can’t spot fix ergonomic problems. It is all or nothing. That is why people don’t heal because they try to fix isolated symptoms and not everything that is part of the problem. You may have pain in your wrist but that is only the location of the symptom. The problem is most likely how you are using your whole arm.
Often it is not a single movement that is a problem but a cavalcade of movement issues. You may type with flat fingers, curled fingers, too much pressure, equalized fingers, not enough “up,” radial deviation, you may abduct too much, you might isolate a finger, dorsiflexion, have an isolated elbow or shoulder . . . there are a lot of motions we should not do but we do them because many of us are lazy and unaware.
In the old days, manual typewriters forced us to type with the weight of the arm or, gravity. Today's effortless keyboards have insidiously encouraged us not to use gravity and the fulcrum of the elbow to type and thus, we isolate smaller parts which strain our tendons. There is no such thing as "repetitive stress." There is only improper movement and if you move improperly, all movement is then "repetitive stress."
Imagine casting a fishing pole with just your fingers, you'd probably hurt yourself. Now imagine that only with the wrist. That is better but still not optimal. Now with your elbow. Better. Add the shoulder. Notice how you are now using all the parts of the arm for one movement. No single part is isolated but they all share in the casting, including but not exclusively the fingers. Now as you cast, notice how your feet are planted, how your weight or center of gravity is distributed, your back and abs, notice also the equal and opposite motion required to cast. In order to cast forward you must first cast backward. Typing, too. In order to type down you must first have an up motion. Without it, you will strain your flexor tendons. That is also the most dangerous part of using a mouse. We rest our index finger and long flexor tendon flat on the button and click with no "up" or equal and opposite motion. There is nothing wrong with the mouse, only how we use it.
The laws of physics must be obeyed. Break them and there is a price to pay.