Cause he's not affiliated with any magic tower, and he needs to pass through the test first if he want to join unless the examiner/above people allow him to just join without taking the exam.
Its an insane power gap but the examiner might want to see how other applicants will deal with him too, considering they let becky who have killed other applicants before to join the test again
wait why is that old man an applicant too like that's a legend??? a fucking person written in history??? ISN'T THAT AN INSANE POWER GAP