Human beings can go their entire lives without a basic understanding of what constitutes good nutrition because they are too robust to just die, and this is considered normal - it's considered abnormal to even read the ingredients list unless you have some sort of dietary issue.
The bar for "functioning" is so low. I assume in a lot of cases it isn't even necessarily malicious. They just think eating the shit that zombified them is actually good for you because they still "function".