How come I have to worry about changing the water hardness when most fish are locally bred in my tap water.

Some fish may be locally bred in your tap water, but the number in an average fish shop is so small to be insignificant.
In the last fiscial year 24 million fish were imported into the UK, of those only 2.9 million were wild caught. Singapore, Japan, Israel, Indonesia, and the Czech Republic make up most of the exporting countries, with Singapore supplying the majority.

The fish breeding industry in Singapore is huge, breeding mostly freshwater, but with major investment going into breeding marine species.
The fish farms, which can be anything from a small consolidator through to the huge farms that breed hundreds of species, have access to a range of scientists, and the larger farms have their own research and development team. One thing that they are prioritising is the reuse of water, to reduce the need for 100% daily water changes, and the need to be altering the water to suit the fishes needs. This isn’t to say the big fish farms get it right every time, but there is a lot of work going into to maintaining the water quality as that reduces losses, increases breeding productivity and growth, and over all means more money for the farmer.

Singapore has a huge advantage over a UK breeder, and that is temperature. Many of these tropical fish are bred in large ponds outside, and many of these industries started out that way. Qian Hu one of the larger fish farmers in Singapore started as a pig farm before environmental reforms meant that fish farming was a better option for them. Speaking as someone who has bred fish for many years now my main cost is always electricity for heating the tanks. Yes some more unusual species may be bred by dedicated breeders in your area, and some of these might find their way into the fish shop. But even the large specialist shops that work to find these local breeders say the number of fish they take in from UK breeders is a tiny percentage.

But here’s the added thing. All the breeders I know either choose fish to suit their water, or alter the water to suit their fish. The idea that people are spending thousands a year on their fish, but not spending a tiny bit extra on an RO unit, or other way of getting the correct water is a bit daft.
There are some fish that are adaptable, and whilst they are found in a different water range in the wild, they can survive and even breed in a different water type, the vast majority will thrive if you give them the correct parameters. If you’re going to buy an animal you are making the choice to bring it into your home. It is your responsibility to make that animals life the best it can be.