That is certainly part of it. Another part is that scuba instructors are both teachers and coaches. Most are accomplished divers and pretty good coaches. Where a lot of them fall down is in the classroom portion of the course. Many (perhaps most) don't really know how to teach in a classroom setting and simply read powerpoint slides aloud or repeat information students are having trouble with verbatim from the manual rather than going at it from a different direction so that it makes more sense to the student. Really good instructors can bridge that chasm, but significant numbers cannot. I know that a lot of students and instructors alike dread the classroom parts of the certification courses, meaning that many students don't get much from them anyway and may actually learn better in another way. I have certifications from three different agencies in a very large number of courses, and all but one of my instructors was an uninspiring classroom teacher, and yet great role models and coaches in the water.