From what I've seen it's not as much refusing to leave as much as it is inability and ignorance that cause them to stick around.
I know countless people who go to college, get their degree, then go back to their small hometown and live with their parents unemployed, or underemployed, because they refuse to move away where they can get a real job, real experience, and establish a career. Nothing wrong with living in a small town if you are supporting yourself independently and you are happy. But if you are struggling to get by, not self sufficient, and have the ability to thrive and you are too scared to do so, I think that is stupid. And it is a very common mindset among smalltowners and I just don't get it. I couldn't wait to leave my small town - the thought of being stuck there was depressing.