> have known for hundreds or thousands of years to keep feces away from their food and water

It wasn't that long ago that Chicago had to raise all their buildings a couple meters, to help keep feces away from the people. And in the next generation had to reverse the course of a river, to keep feces away from their drinking water.

So it's certainly not that ingrained, or not since so long ago, or, as most things in life, easier said than done.