If you think that pH or GH aren't so important factors, how do you explain that fishes are much often sick when their are kept in wrong water? And what about fish physiology?
Sure, they are kept in different kind of water in stores. But, they are kept only short time in those stores -rest of their lives they live with new owner. Do you want to keep them in water where they need energy to evenly osmotic pressure or to keep them in water where osmotic pressure is ok and fishes don't need to extra energy to keep o-pressure good. When it takes more energy to survive in e.g. wrong osmotic pressure, the energy is taken from somewhere else, e.g. from fish health. Soft/acid water fishes can tolerate better harder water than basic/hard water fishes can tolerate soft/acid water.
And what comes in breeding... How many years, decades and generations it takes that you have breeded a fish that can tolerate different water than it's great-great...-great-great parents? And how do you managed to do that? Changing osmotic pressure just a little units-by-units when new generation is born? And avoiding in-breeding you need lots of different generation. It's a big question to where I haven't found an answer yet..
So, it's much easier to hobbiest (and fishes are heath too) to choose fishes that can tolerate easily water that you have. Then you don't need to make any changes to water either, softening or hardening etc...