I betcha a lot of the minerals precipitate out after the boil.
I use the water that comes out of the spring up on the hill behind my house
Unfiltered, untreated. No idea what the ph is like, but it makes tasty beer.
If you use bottled water, get spring water, not something that has been reverse osmosised. That filtering will strip out all the minerals you need for happy beer.
The biggest concern for me when brewing with city water is the chlorine and floride and all the crap they add to it. I guess a lot of that boils off...