The same thing happened with kata and kata judging. I learned new kata and became an instructor, polishing and upgrading skills at other camps. In addition to being able to certify others in different kata, I can also certify Judges. Going through the Rank Examiner course at every camp developed my skills to the degree that now I often teach the course. Learning skills and becoming proficient in teaching them has a direct link to judo camp.
Camps are long enough to allow new learning to sink in.
Weekend clinics are too short for certification in refereeing and some kata. Refereeing requires that a real or simulated shiai accompany the course for practical application. This is difficult if not impossible to do with only a few judoka and in a short time frame. Camp allows time for instruction, application, and evaluation. There are sufficient attendees for a shiai at which to train and evaluate referees.