I remember being a kid (8 or 9) and sitting down with my friend at his house with his parents and going through all our candy together to check for danger signs. Sure enough, there was a single-wrapped Reese's peanut-butter cup with a hole in the wrapper, and we opened it up and found a sewing needle stuck inside the peanut-butter cup. (I don't think I'm making this up, but memories being what they are, this might have been a story that was told to me. I do remember vividly the breaking of the peanut-butter cup into two pieces and the needle falling on the table, though, so I'm inclined to believe the memory.)
This would have been in the late 1980s. So, in my case, the fear did not come from the media, but from a personal experience.
So rather than saying, "this never happens, it's just a myth", we should be saying, "this can happen, and let's take steps to protect ourselves".
Anyone remember the Tylenol Poisonings[0]? That was real, inspired copycats, and led to tamper-resistant and tamper-evident packaging. Tragic, but at least we learned from the tragedy.
While it's true that not everyone is out to get you, at the same time it's foolish to think that no-one is out to get you.
[0] https://en.wikipedia.org/wiki/Chicago_Tylenol_murders