Simply put, our water is VERY bad. No one wants to talk about it and as long as it comes out of your faucet no one really cares.
Either drink your water and die from the many, poisons, viruses and bugs that are in it.
Or, dump a bunch of chemicals in it, playing it off as good and safe for you. Preventing that nastiness from getting you.
We drink so much water and due to the size of our population, filtration on a massive scale is VERY expensive. So they play it down, toss some bleach and fluoride into it.
Move on to other things. By the time you get sick from it it will be blamed on something else.