I've only met one in person, outside of B&M knife shops and shows.
Got into a conversation with a white-haired white-bearded gentleman at work one day, we ended up surreptitiously comparing EDC in the office. He didn't like thumb studs, didn't trust them wet, and so favored Spyderco. Goes to the local shows (more than I do). We don't live close, don't hang out. Don't know if it's a coincidence, but of the four CompSci PhDs I've worked with, he was the only one that could do more than talk convincingly. He could actually program, and well. It was a shock.
IMHO, knife nuts are born. My parents would never have considered carrying such a thing (unlike my grandfathers, both sides). I grew up with ZERO influence or experience in that direction, bought my first jack-knife with saved allowance at maybe seven years old, and have carried ever since except where it's prohibited by law (and once in a while where it was). Got it trouble a couple of times for carrying, my parents were appalled and humiliated.
My younger brother seems to have gotten some of it, my parents were appalled all over again.

At least in my own case, I can pretty safely say that it wasn't environment.