Unfortunately, Religions are often very controlling… and how they do that controlling is often though bent truths, or outright fabrications.
I try to be very respectful of others beliefs, just as i’d want them to be respectful of mine… but it’s hard to as, how @johnpowell put it, “religious nuts” ruin it for the decent religious folk. This is an example of that.
Discarding religions and we still have superstitions. Even without religion involved, people believe in a lot of daft things. If you break a mirror, you get 7 years bad luck. That kind of thing. A mirror is nothing more than a sheet of glass with a reflective backing.
When you take both of these into account, I’m sure it can account for that myth.