Filtering water in the backcountry is extremely easy with products like the Sawyer Mini ($25), which you just screw to the top of a standard water bottle and drink through. It’s stupid to take the risk when the precautions are so easy and so cheap. And it’s unbelievable that people in civilization, with abundant access to clean water, would actually pay a premium for dirty water.
Yes, primitive people drank water straight from streams. They built up resistance to a lot of the pathogens, but they also had lots of parasites and diseases, and didn’t regularly live into their 80s, so I’m not sure why you’d model your food and water safety on theirs.
[1] Viruses are a problem in much of the world too. Thankfully, streams in the US don’t generally have viruses in them, and the most popular filters for hikers don’t filter out all viruses. But for the extra careful, chlorine dioxide drops and some pump filters do.