Plausible deniability. The real part of the security clearance is the background check they perform, including the interviews. If they find out from some secret source that you immigrated from North Korea, they won’t tell you they figured that out. They’ll just tell you that you didn’t pass the polygraph and send you home. Your North Korean handler will report back that they need to train future spies how to defeat the polygraph, but fail to close the actual hole in their security.