I can't tell you how many children and adults I have come across who tell me that they just can't sing. I used to teach deaf kids how to sing.
Children believe what they are told, and it carries on in life.
It has a bigger impact than people would ever imagine. The truth is, there are many ways to learn how to sing. Not everyone is opera material, but not everyone wants to be either. People just say it because it's something to say. Most of the time they don't even mean it.