You also have to consider some selection bias.... If a kid has no perfectionism, anxiety, sensitivity, sensory issues or emotional difficulties... how likely is it that his parents will bother finding out that he's (happily) gifted?
In our case, DS is happy, calm and even-keeled. No sensory issues, not much anxiety (no more than anyone else in the family, but it is something I keep an eye on), no anger management issues or other emotional or behavioral difficulties, no potty training issues, tests well, reads social cues and subtleties like a psychic, takes on challenges without blinking, turns on a dime as needed... He was clingy as a baby, but at the time I thought that was normal.
I don't know why he doesn't have issues, except that we only tested (IQ) because of some early verbal quirks, and (achievement) because of the homeschool laws here. I could see never having realized he was anything more than just a good student, if a few little things had been different.
