“I live in a city"
Your water supply is pumped from an aquifer or lake and although it is treated by the city approximately 2% has been treated for in-home use. The remaining 98% is for industrial or municipal use. The City may have treated this household water to a very high degree; that meets or exceed the EPA standards however; it would still need to travel through miles of pipe to reach your home. 
This is why more water treatment is necessary when water enters your home.