Not true at all. The vast majority of wild, moving water is likely safe in the USA. A study from a few years back tested hundreds of streams in the Sierra. They found crypto or giardia in very few of them, mostly down towards the foothills near agriculture impacts. That doesn't mean don't treat. I've had giardia when I was a boyscout. It is NOT FUN. Also keep an eye on the source if it's downstream from mining impacts. Huge issue in Montana and Colorado. I almost always drink wild water up high. Biggest issue in the wild is personal hygiene. Way more likely to get ill from dirty human stuff. Filtration is smart. Low likelihood of illness but said illness can create a survival situation.
Not sure how old you are, but when I was in the Boy Scouts in the early to mid 80s, we were told to ONLY trust wild water sources if they are filtered. I still have my First Need, and use it when I need to get water from untrusted sources. As you mentioned, Giardia is NOT fun. That is why filling up from known good, generally municipal sources is a good idea. If you can't, filter, and even then, sanitize with iodine drops. But generally speaking I will NOT introduce untrusted water to my onboard water system.