just that they do this to his tankiness(shield/ult-heal), so it hits mostly rather gimicjy spots(if you recive the shield, your enemies are usually stunned and don't deal damage anway, ult is on a high cd and situational) and doesn't impact his trades all that much.
If you nerf the damage base values and increase the ratios, you recive a very binary champion that is incredible opressive if ahead, because a lead means more ap means way more damage if the ratios get increased, but totally usless if behind cause less gold means less ap means poor damage with worse basestats.
also, low base stats and high scaleings means weaker earlygame, so fizz probably would loose killpresure vs ranged laners and thus get buillied ery hard, which delays his itmes and with that the ap that he desperatly needs...midlane fizz would be pretty dead
what they could do is changeing his ult-debuff from 20% increased damage to 20% increased magic damage