All the things that go down the drain and end up at the waste water treatment plant are not removed there. Some of the industrial byproducts that end up in sewers, the agricultural chemicals that runoff farmland, and pharmaceuticals that pass through our bodies all can end up in our streams and lakes.
Water treatment plants do a good job of killing the viruses and bacteria that cause waterborne diseases such as typhoid, cholera, and dysentery. But dealing with chemicals seems more complicated for regulators. Only recently PFAS was discovered in drinking water, but it had been there for decades. More than likely there are other chemicals not yet detected.
Scientists don’t have the research facilities and money necessary to study what the complex mixtures of chemicals do to human health.