Sadly, it's more typical than you might think. When I worked at an FBO, I was absolutely blown away by the number of people who I'd see pass their checkride and then never see them at the airport again. In fact, I think it was the majority.
I think part of this is that we might even push too much caution. I may sound like a loose cannon for saying this, but I think the vast majority of people will be cautious about flying by default, and those who won't (the "type a personalities") will kill themselves no matter how much we talk about caution. If we talk about caution to the extent we think is required to save the unsaveable, we may be making flying scary enough for that majority that they never feel comfortable with flying without a CFI aboard, and since that's generally not practical for the things that flying is useful for (that Christmas trip to visit Grandma, etc.), they simply give it up, check the box on their bucket list, and move on.