Same reason that a phd isn't really a path to being a tenured professor. Yes, it is the only way to get a tenured position, but the vast majority of people try

Same reason that a phd isn't really a path to being a tenured professor. Yes, it is the only way to get a tenured position, but the vast majority of people trying don't get one. And not because they didn't work hard or even because they aren't qualified.