I would believe it's an education problem, in that we teach things like recursion and local variables (the fact that the same variable can have different values

I would believe it's an education problem, in that we teach things like recursion and local variables (the fact that the same variable can have different values in a nested call to a function) in intro CS classes but we don't really teach how post-NULL languages handle optionals (Option, Maybe, etc.). But it's something that should be part of an effective programmer's knowledge. It's not too unlike buffer overflows—all the people writing UNIX until 1988 were skilled, educated, experienced people, but at the time none of them were really aware just how bad buffer overflows were.