If you go out into the Rockies or Appalachians you can drink most surface water untreated
Sure, there are many places here in Australia where you can still drink from a mountain stream. However the vast majority of our ancestors over the last 5-10,000yrs did not live in next to a pristine mountain stream. They lived in towns and villages with open sewers running thru the streets and into the waterways. The local water was not fit for human consumption, people did not drink it because even though they knew nothing of germs they knew that dirty water could/would give them cholera and/or dysentery, they also knew that turning it into beer/wine prevented that agonising fate from happening to them. .